Fix type warnings in API and helpers.

sdk/lib/_internal/compiler/implementation/lib/interceptors.dart

Index: sdk/lib/_internal/compiler/implementation/lib/interceptors.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/interceptors.dart b/sdk/lib/_internal/compiler/implementation/lib/interceptors.dart
index 927e82a17ffb89a06da7cfb6e09b2fbbe8508dab..d5dccf6a8f8e4097e6ece06683698d5cf8b1ca1d 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/interceptors.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/interceptors.dart
@@ -5,7 +5,8 @@
 library _interceptors;
 
 import 'dart:collection';
-import 'dart:_collection-dev';
+import 'dart:_collection-dev' hide Symbol;
+import "dart:_collection-dev" as _symbol_dev show Symbol;
 import 'dart:_js_helper' show allMatchesInStringUnchecked,
                               Null,
                               JSSyntaxRegExp,
@@ -30,6 +31,17 @@ part 'js_array.dart';
 part 'js_number.dart';
 part 'js_string.dart';
 
+String _symbolToString(Symbol symbol) => _symbol_dev.Symbol.getName(symbol);
+
+_symbolMapToStringMap(Map<Symbol, dynamic> map) {
+  if (map == null) return null;
+  var result = new Map<String, dynamic>();
+  map.forEach((Symbol key, value) {
+    result[_symbolToString(key)] = value;
+  });
+  return result;
+}
+
 /**
  * Get the interceptor for [object]. Called by the compiler when it needs
  * to emit a call to an intercepted method, that is a method that is
@@ -277,10 +289,11 @@ abstract class Interceptor {
   String toString() => Primitives.objectToString(this);
 
   dynamic noSuchMethod(Invocation invocation) {
-    throw new NoSuchMethodError(this,
-                                invocation.memberName,
-                                invocation.positionalArguments,
-                                invocation.namedArguments);
+    throw new NoSuchMethodError(
+        this,
+        _symbolToString(invocation.memberName),
+        invocation.positionalArguments,
+        _symbolMapToStringMap(invocation.namedArguments));
   }
 
   Type get runtimeType => getRuntimeType(this);