dart2js: build metadata-types lazily in the new emitter.

pkg/compiler/lib/src/js_emitter/new_emitter/model_emitter.dart

Index: pkg/compiler/lib/src/js_emitter/new_emitter/model_emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/new_emitter/model_emitter.dart b/pkg/compiler/lib/src/js_emitter/new_emitter/model_emitter.dart
index 081270bbf351ae88e7fc01d80dc49a54dc98b8d1..8902af8448a276b3bbfc9e5b51a56b29f560ce78 100644
--- a/pkg/compiler/lib/src/js_emitter/new_emitter/model_emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/new_emitter/model_emitter.dart
@@ -200,6 +200,7 @@ class ModelEmitter {
            backend.emitter.staticFunctionAccess(backend.getCyclicThrowHelper()),
        'outputContainsConstantList': program.outputContainsConstantList,
        'embeddedGlobals': emitEmbeddedGlobals(program),
+       'readMetadataTypeFunction': readMetadataTypeFunction,
        'staticNonFinals':
             emitStaticNonFinalFields(fragment.staticNonFinalFields),
        'operatorIsPrefix': js.string(namer.operatorIsPrefix),
@@ -420,6 +421,31 @@ class ModelEmitter {
     return new js.Property(js.string(GET_TYPE_FROM_NAME), function);
   }
 
+  static final metadataTypeComment =
+      "Types are non-evaluated and must be compiled at first use. "
+      "Compiled strings are guaranteed not to be strings, and it's thus safe "
+      "to use a type-test to determine if a type has already been compiled.";
+
+  static final String readMetadataTypeName = "readMetadataType";
+
+  js.Statement get readMetadataTypeFunction {
+    return js.js.statement('''function $readMetadataTypeName(index) {
+      // $metadataTypeComment.

[sra1, 2015/05/07 03:39:30]

Just put the comment here.

[floitsch, 2015/05/07 04:08:59]

Done.

+      var type = #typesAccess[index];
+      if (typeof type == 'string') {
+        type = expressionCompile(type);
+        #typesAccess[index] = type;
+      }
+      return type;
+    }''', {"typesAccess": generateEmbeddedGlobalAccess(TYPES)});
+  }
+
+  js.Template get templateForReadType {
+    // TODO(floitsch): make sure that no local variable shadows the access to
+    // the readMetadataType function.
+    return js.js.expressionTemplateFor('$readMetadataTypeName(#)');
+  }
+
   List<js.Property> emitMetadata(Program program) {
 
     List<js.Property> metadataGlobals = <js.Property>[];
@@ -435,7 +461,9 @@ class ModelEmitter {
     List<String> types =
         program.metadataTypes[program.fragments.first.outputUnit];
     if (types == null) types = <String>[];
-    metadataGlobals.add(createGlobal(types, TYPES));
+    // The strings in the array are already pretty-printed JS expressions.
+    js.Expression unparsedTypes = js.stringArray(types);
+    metadataGlobals.add(new js.Property(js.string(TYPES), unparsedTypes));
 
     return metadataGlobals;
   }
@@ -464,7 +492,7 @@ class ModelEmitter {
 
     js.LiteralString immediateString = unparse(compiler, immediateCode);
 
-    js.Expression deferredTypes = types == null
+    js.Expression deferredTypes = (types == null)
         ? js.string("[]")
         : js.string("[${types.join(",")}]");
 
@@ -1162,6 +1190,13 @@ function parseFunctionDescriptor(proto, name, descriptor, typesOffset) {
   // Initialize globals.
   #embeddedGlobals;
 
+  function expressionCompile(__s__) {
+    'use strict';
+    return eval('(' + __s__ + ')');
+  }
+
+  #readMetadataTypeFunction;
+
   // TODO(floitsch): this order means that native classes may not be
   // referenced from constants. I'm mostly afraid of things like using them as
   // generic arguments (which should be fine, but maybe there are other