Extract DiagnosticReporter implementation from Compiler.

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 af554558ebedc39e5efd908eae704929202b8ddb..e0a9b03bec7de32797f526f4af0fef9661fd6c5f 100644
--- a/pkg/compiler/lib/src/js_emitter/metadata_collector.dart
+++ b/pkg/compiler/lib/src/js_emitter/metadata_collector.dart
@@ -145,6 +145,7 @@ class MetadataCollector implements jsAst.TokenFinalizer {
 
   JavaScriptBackend get _backend => _compiler.backend;
   TypeVariableHandler get _typeVariableHandler => _backend.typeVariableHandler;
+  DiagnosticReporter get reporter => _compiler.reporter;
 
   bool _mustEmitMetadataFor(Element element) {
     return _backend.mustRetainMetadata &&
@@ -159,13 +160,13 @@ class MetadataCollector implements jsAst.TokenFinalizer {
   /// mirrors_patch to implement DeclarationMirror.metadata.
   jsAst.Fun buildMetadataFunction(Element element) {
     if (!_mustEmitMetadataFor(element)) return null;
-    return _compiler.withCurrentElement(element, () {
+    return reporter.withCurrentElement(element, () {
       List<jsAst.Expression> metadata = <jsAst.Expression>[];
       for (MetadataAnnotation annotation in element.metadata) {
         ConstantValue constant =
             _backend.constants.getConstantValueForMetadata(annotation);
         if (constant == null) {
-          _compiler.internalError(annotation, 'Annotation value is null.');
+          reporter.internalError(annotation, 'Annotation value is null.');
         } else {
           metadata.add(_emitter.constantReference(constant));
         }
@@ -195,7 +196,7 @@ class MetadataCollector implements jsAst.TokenFinalizer {
     ConstantValue constant =
         _backend.constants.getConstantValueForMetadata(annotation);
     if (constant == null) {
-      _compiler.internalError(annotation, 'Annotation value is null.');
+      reporter.internalError(annotation, 'Annotation value is null.');
       return null;
     }
     return _addGlobalMetadata(_emitter.constantReference(constant));
@@ -255,7 +256,7 @@ class MetadataCollector implements jsAst.TokenFinalizer {
     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(
+      reporter.internalError(
           NO_LOCATION_SPANNABLE, 'reified types should not be strings.');
     }
 
@@ -281,7 +282,7 @@ class MetadataCollector implements jsAst.TokenFinalizer {
   }
 
   List<jsAst.DeferredNumber> computeMetadata(FunctionElement element) {
-    return _compiler.withCurrentElement(element, () {
+    return reporter.withCurrentElement(element, () {
       if (!_mustEmitMetadataFor(element)) return const <jsAst.DeferredNumber>[];
       List<jsAst.DeferredNumber> metadata = <jsAst.DeferredNumber>[];
       for (MetadataAnnotation annotation in element.metadata) {