Improvements to build-js.sh and JS API (#679)

* Adds command-line argument to build-js.sh for specifying the location of Emscripten. Also spreads out program arguments on separate lines

* Exposes WasmBinaryWriter and BufferWithRandomAccess in the JS API to allow writing WASM modules and access the produced bytes via the JS API

* Updates bin/binaryen.js and bin/wasm.js from changes to build-js.sh

* Adds exclude patterns to .gitignore for some files generated by build-js.sh and .DS_Store for macOS

* Changes build-js.sh to use EMSCRIPTEN env var instead of a command-line argument to provide the path to emscripten

* Improvements to JS builds

- Adds -g flag to build-js.sh that will build unoptimized and unmangled versions of binaryen.js and wasm.js (output has a "-g.js" suffix to allow co-existence with regular optimized builds).
- Enables closure compiler for non-debug builds
- Adds browser test for s-expression parser + WASM code gen (requires wasm to be enabled in the browser.)
- Adds iterator interface to BufferWithRandomAccess (when Symbol.iterator is available)
- Adds toArrayBuffer to BufferWithRandomAccess (when TypedArray is available)
- Adds compileWast(sourceText :string) :ArrayBuffer to the module, parsing & compiling s-expression code to a WASM module
- Changes the way binaryen.js is exported to allow usage in CommonJS, AMD and UMD envionments.

* Expose "Binaryen" global in a better way to work with a.js generated by check.py

* Fix to binaryen.js to only export a global variable when running the test (a.js) while inside a module (avoids polluting global in e.g. nodejs). Also fixes a spelling mistake.

* Better "no WASM detected" message in test/binaryen.js/browser.html

* Small change to error message in build-js.sh where $EMSCRIPTEN does not point to a directory

* Changes emcc args in build-js.sh after investingating a large number of argument combinations. Also adds a browser benchmark. The result of emcc arguments and the effect on performance is summarized in this doc: https://gist.github.com/rsms/e33c61a25a31c08260161a087be03169

* Enable inferring emscripten path by looking in PATH when EMSCRIPTEN is not set in env

4 files changed, 98 insertions, 1 deletions

diff --git a/src/js/binaryen.idl b/src/js/binaryen.idl
index 6a1d63d8a..b7244fbe2 100644
--- a/src/js/binaryen.idl
+++ b/src/js/binaryen.idl
@@ -30,7 +30,18 @@ interface ModuleInstance {
   [Value] Literal callExport([Ref] Name name, [Ref] LiteralList arguments);
 };
 
-[Prefix="ModuleInstance::"]
+
+interface BufferWithRandomAccess {
+  void BufferWithRandomAccess(boolean debug);
+  unsigned long size();
+  [Operator="[]"] octet at(unsigned long index);
+};
+
+interface WasmBinaryWriter {
+  void WasmBinaryWriter(Module m, [Ref] BufferWithRandomAccess outbuf, boolean debug);
+  void write();
+};
+
 interface LiteralList {
   void LiteralList();
 
diff --git a/src/js/binaryen.js-extended.js b/src/js/binaryen.js-extended.js
new file mode 100644
index 000000000..74e3e4bbe
--- /dev/null
+++ b/src/js/binaryen.js-extended.js
@@ -0,0 +1,56 @@
+// Extended API
+
+// Makes the internal buffer iterable so we can use Uint8Array.from
+if (typeof Symbol !== 'undefined' && Symbol.iterator) {
+  Module['BufferWithRandomAccess'].prototype[Symbol.iterator] = function() {
+    var b = this, i = 0, L = b.size();
+    return {
+      next: function() {
+        if (i < L) {
+          return { done: false, value: b.at(i++) }
+        } else {
+          return { done: true }
+        }
+      }
+    }
+  };
+}
+
+// Converts (copies) a BufferWithRandomAccess to native ArrayBuffer
+if (typeof Uint8Array !== 'undefined') {
+  Module['BufferWithRandomAccess'].prototype['toArrayBuffer'] = function() {
+    return Uint8Array['from'](this).buffer; // :ArrayBuffer
+  };
+}
+
+// Parse and compile S-expression text syntax into WASM code.
+// When running in NodeJS, this functions returns a Buffer instead of ArrayBuffer.
+Module['compileWast'] = function compileWast(sourceText /*:string*/) /*:ArrayBuffer*/ {
+  var parser = new Module['SExpressionParser'](sourceText);
+
+  // console.log('s-expr dump:');
+  // parser.get_root().dump();
+
+  var s_module = parser.get_root().getChild(0);
+  // console.log('s_module', s_module)
+
+  // strange API for creating WASM `module` from S-expression AST
+  var module = new Module['Module']();
+  new Module['SExpressionWasmBuilder'](module, s_module);
+
+  // console.log('module:', module);
+  // this.WasmPrinter.prototype.printModule(module);
+
+  // emit WASM
+  var debug = false;
+  var buf0 = new Module['BufferWithRandomAccess'](debug);
+  var wasmWriter = new Module['WasmBinaryWriter'](module, buf0, debug);
+  wasmWriter.write();
+
+  if (ENVIRONMENT_IS_NODE) {
+    return Buffer['from'](Uint8Array['from'](buf0));
+  } else {
+    return buf0['toArrayBuffer']();
+  }
+};
+
diff --git a/src/js/binaryen.js-post.js b/src/js/binaryen.js-post.js
new file mode 100644
index 000000000..7389f5afe
--- /dev/null
+++ b/src/js/binaryen.js-post.js
@@ -0,0 +1,29 @@
+return Module;
+};
+if (typeof exports != 'undefined') {
+  (function(){
+    var a = Binaryen();
+    if (typeof module === 'object') {
+      module.exports = a;
+    } else {
+      for (var k in a) {
+        exports[k] = a[k];
+      }
+    }
+  })();
+}
+(typeof window !== 'undefined' ? window :
+ typeof global !== 'undefined' && (
+  typeof process === 'undefined' ||
+  
+  // Note: We must export "Binaryen" even inside a CommonJS/AMD/UMD module
+  // space because check.py generates a.js which requires Binaryen global var
+  ( process.argv &&
+    Array.isArray(process.argv) &&
+    process.argv[1] &&
+    process.argv[1].substr(-5) === '/a.js'
+  )
+
+ ) ? global :
+ this
+)['Binaryen'] = Binaryen;
diff --git a/src/js/binaryen.js-pre.js b/src/js/binaryen.js-pre.js
new file mode 100644
index 000000000..ceb25f165
--- /dev/null
+++ b/src/js/binaryen.js-pre.js
@@ -0,0 +1 @@
+var Binaryen = function(Module){