Delete dart2js specific private library code

lib/runtime/dart/core.js

 var core = dart.defineLibrary(core, {});
 var _js_helper = dart.lazyImport(_js_helper);
 var _internal = dart.lazyImport(_internal);
 var collection = dart.lazyImport(collection);
 var _interceptors = dart.lazyImport(_interceptors);
 var math = dart.lazyImport(math);
 var convert = dart.lazyImport(convert);
 (function(exports, _js_helper, _internal, collection, _interceptors, math, convert) {
   'use strict';
   class Object {
@@ skipping 40 matching lines @@
   });
   class JsPeerInterface extends Object {
     JsPeerInterface(opts) {
       let name = opts && 'name' in opts ? opts.name : null;
       this.name = name;
     }
   }
   dart.setSignature(JsPeerInterface, {
     constructors: () => ({JsPeerInterface: [JsPeerInterface, [], {name: String}]})
   });
-  class SupportJsExtensionMethod extends Object {
-    SupportJsExtensionMethod() {
+  class SupportJsExtensionMethods extends Object {
+    SupportJsExtensionMethods() {
     }
   }
-  dart.setSignature(SupportJsExtensionMethod, {
-    constructors: () => ({SupportJsExtensionMethod: [SupportJsExtensionMethod, []]})
+  dart.setSignature(SupportJsExtensionMethods, {
+    constructors: () => ({SupportJsExtensionMethods: [SupportJsExtensionMethods, []]})
   });
   class Deprecated extends Object {
     Deprecated(expires) {
       this.expires = expires;
     }
     toString() {
       return `Deprecated feature. Will be removed ${this.expires}`;
     }
   }
   dart.setSignature(Deprecated, {
@@ skipping 3654 matching lines @@
   function _symbolMapToStringMap(map) {
     if (map == null)
       return null;
     let result = Map$(String, Object).new();
     map.forEach(dart.fn((key, value) => {
       result.set(_symbolToString(key), value);
     }, Object, [Symbol, Object]));
     return result;
   }
   dart.fn(_symbolMapToStringMap, Object, [Map$(Symbol, Object)]);
-  class SupportJsExtensionMethods extends Object {
-    SupportJsExtensionMethods() {
-    }
-  }
-  dart.setSignature(SupportJsExtensionMethods, {
-    constructors: () => ({SupportJsExtensionMethods: [SupportJsExtensionMethods, []]})
-  });
   class _ListConstructorSentinel extends Object {
     _ListConstructorSentinel() {
     }
   }
   dart.setSignature(_ListConstructorSentinel, {
     constructors: () => ({_ListConstructorSentinel: [_ListConstructorSentinel, []]})
   });
   // Exports:
   exports.Object = Object;
   exports.JsName = JsName;
   exports.JsPeerInterface = JsPeerInterface;
-  exports.SupportJsExtensionMethod = SupportJsExtensionMethod;
+  exports.SupportJsExtensionMethods = SupportJsExtensionMethods;
   exports.Deprecated = Deprecated;
   exports.deprecated = deprecated;
   exports.override = override;
   exports.proxy = proxy;
   exports.bool = bool;
   exports.Comparator$ = Comparator$;
   exports.Comparator = Comparator;
   exports.Comparable$ = Comparable$;
   exports.Comparable = Comparable;
   exports.DateTime = DateTime;
@@ skipping 104 matching lines @@
   exports.Sink = Sink;
   exports.StackTrace = StackTrace;
   exports.Stopwatch = Stopwatch;
   exports.String = String;
   exports.RuneIterator = RuneIterator;
   exports.StringBuffer = StringBuffer;
   exports.StringSink = StringSink;
   exports.Symbol = Symbol;
   exports.Type = Type;
   exports.Uri = Uri;
-  exports.SupportJsExtensionMethods = SupportJsExtensionMethods;
 })(core, _js_helper, _internal, collection, _interceptors, math, convert);