EM_JS binaryen support (#1410)

* Emit EM_JS metadata

* Include s2wasm-style em_js support

* Change em_js metadata to be keyed on name

* Add testcase for em_js, don't always emit emJsFuncs metadata

* Better error handling for unexpectedly-formatted __em_js__ functions

1 files changed, 8 insertions, 4 deletions

diff --git a/src/emscripten-optimizer/istring.h b/src/emscripten-optimizer/istring.h
index 74b2c1e4b..8c77c4499 100644
--- a/src/emscripten-optimizer/istring.h
+++ b/src/emscripten-optimizer/istring.h
@@ -137,14 +137,18 @@ struct IString {
   bool is() const     { return str != nullptr; }
   bool isNull() const { return str == nullptr; }
 
-  bool startsWith(const char *prefix) const {
+  const char* stripPrefix(const char *prefix) const {
     const char *ptr = str;
     while (true) {
-      if (*prefix == 0) return true;
-      if (*ptr == 0) return false;
-      if (*ptr++ != *prefix++) return false;
+      if (*prefix == 0) return ptr;
+      if (*ptr == 0) return nullptr;
+      if (*ptr++ != *prefix++) return nullptr;
     }
   }
+
+  bool startsWith(const char *prefix) const {
+    return stripPrefix(prefix) != nullptr;
+  }
 };
 
 } // namespace cashew