Make new of an ambiguous a warning.

pkg/compiler/lib/src/resolution/constructors.dart

Index: pkg/compiler/lib/src/resolution/constructors.dart
diff --git a/pkg/compiler/lib/src/resolution/constructors.dart b/pkg/compiler/lib/src/resolution/constructors.dart
index 26e7b48b21e7c040fda020998fcc0a23b922233a..ce4e93c191ccbc31d87e1924d2f867f050560b3e 100644
--- a/pkg/compiler/lib/src/resolution/constructors.dart
+++ b/pkg/compiler/lib/src/resolution/constructors.dart
@@ -485,18 +485,19 @@ class ConstructorResolver extends CommonResolverVisitor<ConstructorResult> {
       MessageKind kind,
       Map arguments,
       {bool isError: false,
-       bool missingConstructor: false}) {
+       bool missingConstructor: false,
+       List<DiagnosticMessage> infos: const <DiagnosticMessage>[]}) {
     if (missingConstructor) {
       registry.registerFeature(Feature.THROW_NO_SUCH_METHOD);
     } else {
       registry.registerFeature(Feature.THROW_RUNTIME_ERROR);
     }
+    DiagnosticMessage message =
+        reporter.createMessage(diagnosticNode, kind, arguments);
     if (isError || inConstContext) {
-      reporter.reportErrorMessage(
-          diagnosticNode, kind, arguments);
+      reporter.reportError(message, infos);
     } else {
-      reporter.reportWarningMessage(
-          diagnosticNode, kind, arguments);
+      reporter.reportWarning(message, infos);
     }
     ErroneousElement error = new ErroneousConstructorElementX(
         kind, arguments, name, enclosing);
@@ -616,8 +617,7 @@ class ConstructorResolver extends CommonResolverVisitor<ConstructorResult> {
     if (send != null) {
       // The type name is of the form [: prefix . identifier :].
       String name = send.receiver.asIdentifier().source;
-      Element element = resolver.reportLookupErrorIfAny(
-          lookupInScope(reporter, send, resolver.scope, name), node, name);
+      Element element = lookupInScope(reporter, send, resolver.scope, name);
       if (element != null && element.isPrefix) {
         prefix = element;
       }
@@ -666,10 +666,8 @@ class ConstructorResolver extends CommonResolverVisitor<ConstructorResult> {
 
   ConstructorResult visitIdentifier(Identifier node) {
     String name = node.source;
-    Element element = resolver.reportLookupErrorIfAny(
-        lookupInScope(reporter, node, resolver.scope, name), node, name);
+    Element element = lookupInScope(reporter, node, resolver.scope, name);
     registry.useElement(node, element);
-    // TODO(johnniwinther): Change errors to warnings, cf. 11.11.1.
     return constructorResultForElement(node, name, element);
   }
 
@@ -691,6 +689,15 @@ class ConstructorResolver extends CommonResolverVisitor<ConstructorResult> {
           resolver.enclosingElement, name,
           MessageKind.CANNOT_RESOLVE,
           {'name': name});
+    } else if (element.isAmbiguous) {
+      AmbiguousElement ambiguous = element;
+      return reportAndCreateErroneousConstructorElement(
+          node,
+          ConstructorResultKind.INVALID_TYPE, null,
+          resolver.enclosingElement, name,
+          ambiguous.messageKind,
+          ambiguous.messageArguments,
+          infos: ambiguous.computeInfos(resolver.enclosingElement, reporter));
     } else if (element.isMalformed) {
       return constructorResultForErroneous(node, element);
     } else if (element.isClass) {