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

pkg/compiler/lib/src/js_emitter/metadata_collector.dart

Index: pkg/compiler/lib/src/js_emitter/metadata_collector.dart
diff --git a/pkg/compiler/lib/src/js_emitter/metadata_collector.dart b/pkg/compiler/lib/src/js_emitter/metadata_collector.dart
index bd4ab2977a33f45cc8051c6ee5843a4bb8377247..cad4338308896d7dfdacf6b2f3e3105986b5d73e 100644
--- a/pkg/compiler/lib/src/js_emitter/metadata_collector.dart
+++ b/pkg/compiler/lib/src/js_emitter/metadata_collector.dart
@@ -112,6 +112,12 @@ class MetadataCollector {
               return _backend.isAccessibleByReflection(typedef.element);
             });
 
+    if (representation is jsAst.LiteralString) {
+      // We don't want the representation to be a string, since we use
+      // strings as indicator for non-initialized types in the lazy emitter.
+      _compiler.internalError(
+          NO_LOCATION_SPANNABLE, 'reified types should not be strings.');
+    }
     return addTypeInOutputUnit(
         jsAst.prettyPrint(representation, _compiler).getText(), outputUnit);
   }
@@ -133,8 +139,9 @@ class MetadataCollector {
     }
     return _typesMap[outputUnit].putIfAbsent(compiledType, () {
 
-      if (types[outputUnit] == null)
+      if (types[outputUnit] == null) {
         types[outputUnit] = <String>[];
+      }
 
       types[outputUnit].add(compiledType);
       return types[outputUnit].length - 1;