Some fixes for lazy JS 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 b6171c16c923cc31fe9a5e9edc6582b509be1ca5..d71474d601c95219b0866dcdccce017a411e98b6 100644
--- a/pkg/dev_compiler/lib/js/es6/dart_sdk.js
+++ b/pkg/dev_compiler/lib/js/es6/dart_sdk.js
@@ -1207,10 +1207,20 @@ dart.tagComputed = function(value, compute) {
 dart.tagLazy = function(value, compute) {
   dart.defineLazyProperty(value, dart._runtimeType, {get: compute});
 };
+dart._warn = function(arg) {
+  console.warn(arg);
+};
 const _jsTypeCallback = Symbol('_jsTypeCallback');
 const _rawJSType = Symbol('_rawJSType');
+const _dartName = Symbol('_dartName');
 dart._isInstanceOfLazyJSType = function(o, t) {
   if (t[_jsTypeCallback] != null) {
+    if (t[_rawJSType] == null) {
+      let expected = t[_dartName];
+      let actual = dart.typeName(dart.getReifiedType(o));
+      dart._warn(dart.str`Cannot find native JavaScript type (${expected}) ` + dart.str`to type check ${actual}`);
+      return true;
+    }
     return dart.is(o, t[_rawJSType]);
   }
   if (o == null) return false;
@@ -1218,6 +1228,12 @@ dart._isInstanceOfLazyJSType = function(o, t) {
 };
 dart._asInstanceOfLazyJSType = function(o, t) {
   if (t[_jsTypeCallback] != null) {
+    if (t[_rawJSType] == null) {
+      let expected = t[_dartName];
+      let actual = dart.typeName(dart.getReifiedType(o));
+      dart._warn(dart.str`Cannot find native JavaScript type (${expected}) ` + dart.str`to type check ${actual}`);
+      return o;
+    }
     return dart.as(o, t[_rawJSType]);
   }
   if (o == null) return null;
@@ -1252,6 +1268,9 @@ dart._initialize2 = function() {
   dart.LazyJSType.prototype.as = function as_T(object) {
     return dart._asInstanceOfLazyJSType(object, this);
   };
+  dart.LazyJSType.prototype._check = function check_T(object) {
+    return dart._asInstanceOfLazyJSType(object, this);
+  };
 };
 dart._functionType = function(definite, returnType, args, extra) {
   if (args === void 0 && extra === void 0) {
@@ -2214,7 +2233,6 @@ dart.Dynamic = class Dynamic extends dart.TypeRep {
     return 'dynamic';
   }
 };
-const _dartName = Symbol('_dartName');
 dart.LazyJSType = class LazyJSType extends core.Object {
   new(jsTypeCallback, dartName) {
     this[_jsTypeCallback] = jsTypeCallback;