Fix some errors around []= in core types.

pkg/dev_compiler/lib/js/es6/dart_sdk.js

Index: pkg/dev_compiler/lib/js/es6/dart_sdk.js
diff --git a/pkg/dev_compiler/lib/js/es6/dart_sdk.js b/pkg/dev_compiler/lib/js/es6/dart_sdk.js
index c99499b52dfa537498441df5b1c3dfec572b6a74..e0d4c583d658208905bfc27a090c954def982386 100644
--- a/pkg/dev_compiler/lib/js/es6/dart_sdk.js
+++ b/pkg/dev_compiler/lib/js/es6/dart_sdk.js
@@ -46,12 +46,14 @@ let LinkedHashMapOfString$Object = () => (LinkedHashMapOfString$Object = dart.co
 let ListOfNameValuePair = () => (ListOfNameValuePair = dart.constFn(core.List$(_debugger.NameValuePair)))();
 let JSArrayOfString = () => (JSArrayOfString = dart.constFn(_interceptors.JSArray$(core.String)))();
 let JSArrayOfJsonMLFormatter = () => (JSArrayOfJsonMLFormatter = dart.constFn(_interceptors.JSArray$(_debugger.JsonMLFormatter)))();
+let JSIndexable = () => (JSIndexable = dart.constFn(_interceptors.JSIndexable$()))();
 let JSArray = () => (JSArray = dart.constFn(_interceptors.JSArray$()))();
 let JSMutableArray = () => (JSMutableArray = dart.constFn(_interceptors.JSMutableArray$()))();
 let JSFixedArray = () => (JSFixedArray = dart.constFn(_interceptors.JSFixedArray$()))();
 let JSExtendableArray = () => (JSExtendableArray = dart.constFn(_interceptors.JSExtendableArray$()))();
 let JSUnmodifiableArray = () => (JSUnmodifiableArray = dart.constFn(_interceptors.JSUnmodifiableArray$()))();
 let ArrayIterator = () => (ArrayIterator = dart.constFn(_interceptors.ArrayIterator$()))();
+let JSIndexableOfString = () => (JSIndexableOfString = dart.constFn(_interceptors.JSIndexable$(core.String)))();
 let MatchToString = () => (MatchToString = dart.constFn(dart.functionType(core.String, [core.Match])))();
 let StringToString = () => (StringToString = dart.constFn(dart.functionType(core.String, [core.String])))();
 let ComparableOfnum = () => (ComparableOfnum = dart.constFn(core.Comparable$(core.num)))();
@@ -3553,8 +3555,12 @@ dart.setSignature(_interceptors.JSBool, {
   constructors: () => ({new: dart.definiteFunctionType(_interceptors.JSBool, [])})
 });
 dart.registerExtension(dart.global.Boolean, _interceptors.JSBool);
-_interceptors.JSIndexable = class JSIndexable extends core.Object {};
-_interceptors.JSMutableIndexable = class JSMutableIndexable extends _interceptors.JSIndexable {};
+_interceptors.JSIndexable$ = dart.generic(E => {
+  class JSIndexable extends core.Object {}
+  dart.addTypeTests(JSIndexable);
+  return JSIndexable;
+});
+_interceptors.JSIndexable = JSIndexable();
 _interceptors.JSObject = class JSObject extends core.Object {};
 _interceptors.JavaScriptObject = class JavaScriptObject extends _interceptors.Interceptor {
   new() {
@@ -3617,6 +3623,7 @@ _interceptors.JSArray$ = dart.generic(E => {
   let SetOfE = () => (SetOfE = dart.constFn(core.Set$(E)))();
   let ArrayIteratorOfE = () => (ArrayIteratorOfE = dart.constFn(_interceptors.ArrayIterator$(E)))();
   let ListMapViewOfE = () => (ListMapViewOfE = dart.constFn(_internal.ListMapView$(E)))();
+  let JSIndexableOfE = () => (JSIndexableOfE = dart.constFn(_interceptors.JSIndexable$(E)))();
   let ETobool = () => (ETobool = dart.constFn(dart.functionType(core.bool, [E])))();
   let ETovoid = () => (ETovoid = dart.constFn(dart.functionType(dart.void, [E])))();
   let EAndEToint = () => (EAndEToint = dart.constFn(dart.functionType(core.int, [E, E])))();
@@ -4174,7 +4181,7 @@ _interceptors.JSArray$ = dart.generic(E => {
   }
   dart.setExtensionBaseClass(JSArray, dart.global.Array);
   dart.addTypeTests(JSArray);
-  JSArray[dart.implements] = () => [ListOfE(), _interceptors.JSIndexable];
+  JSArray[dart.implements] = () => [ListOfE(), JSIndexableOfE()];
   dart.setSignature(JSArray, {
     constructors: () => ({
       new: dart.definiteFunctionType(_interceptors.JSArray$(E), []),
@@ -4259,7 +4266,6 @@ _interceptors.JSMutableArray$ = dart.generic(E => {
       super.new();
     }
   }
-  JSMutableArray[dart.implements] = () => [_interceptors.JSMutableIndexable];
   return JSMutableArray;
 });
 _interceptors.JSMutableArray = JSMutableArray();
@@ -5335,7 +5341,7 @@ _interceptors.JSString = class JSString extends _interceptors.Interceptor {
     return this[index];
   }
 };
-_interceptors.JSString[dart.implements] = () => [core.String, _interceptors.JSIndexable];
+_interceptors.JSString[dart.implements] = () => [core.String, JSIndexableOfString()];
 dart.setSignature(_interceptors.JSString, {
   constructors: () => ({new: dart.definiteFunctionType(_interceptors.JSString, [])}),
   getters: () => ({
@@ -11555,7 +11561,7 @@ dart.setSignature(_js_helper.JSName, {
   constructors: () => ({new: dart.definiteFunctionType(_js_helper.JSName, [core.String])}),
   fields: () => ({name: core.String})
 });
-_js_helper.JavaScriptIndexingBehavior = class JavaScriptIndexingBehavior extends _interceptors.JSMutableIndexable {};
+_js_helper.JavaScriptIndexingBehavior = class JavaScriptIndexingBehavior extends core.Object {};
 _js_helper.TypeErrorImplementation = class TypeErrorImplementation extends core.Error {
   new(value, actualType, expectedType) {
     this.message = dart.str`Type '${actualType}' is not a subtype ` + dart.str`of type '${expectedType}'`;