Cache all wrap_jso results so we preserve identity

sdk/lib/html/dartium/html_dartium.dart

Index: sdk/lib/html/dartium/html_dartium.dart
diff --git a/sdk/lib/html/dartium/html_dartium.dart b/sdk/lib/html/dartium/html_dartium.dart
index 95c43f64b33d9239ec51225fad5095bae41037c2..0f6b5ac5d935ae4aa10a4e0e3d5d67da431b5ee3 100644
--- a/sdk/lib/html/dartium/html_dartium.dart
+++ b/sdk/lib/html/dartium/html_dartium.dart
@@ -1149,6 +1149,12 @@ wrap_jso(jsObject) {
       // JS Interop converted the object to a Dart class e.g., Uint8ClampedList.
       return jsObject;
     }
+
+    // To preserve identity, if we already have a wrapper for this, return it.
+    var existing = jsObject['dartClass_instance'];
+    if (existing != null) {
+      return existing;
+    }
     // Try the most general type conversions on it.
     // TODO(alanknight): We may be able to do better. This maintains identity,
     // which is useful, but expensive. And if we nest something that only
@@ -1165,7 +1171,6 @@ wrap_jso(jsObject) {
     if (__interop_checks) {
       debug_or_assert("constructor != null && jsTypeName.length > 0", constructor != null && jsTypeName.length > 0);
     }
-
     var dartClass_instance;
     if (jsObject.hasProperty('dart_class')) {
       // Got a dart_class (it's a custom element) use it it's already set up.
@@ -1175,6 +1180,7 @@ wrap_jso(jsObject) {
       if (func != null) {
         dartClass_instance = func();
         dartClass_instance.blink_jsObject = jsObject;
+        jsObject['dartClass_instance'] = dartClass_instance;
       }
     }
     return dartClass_instance;
@@ -1190,6 +1196,7 @@ wrap_jso(jsObject) {
   return null;
 }
 
+
 /**
  * Create Dart class that maps to the JS Type that is the JS type being
  * extended using JS interop createCallback (we need the base type of the
@@ -37168,10 +37175,10 @@ class Url extends NativeFieldWrapperClass2 implements UrlUtils {
     if ((blob_OR_source_OR_stream is Blob || blob_OR_source_OR_stream == null)) {
       return _blink.BlinkURL.instance.createObjectURL_Callback_1_(unwrap_jso(blob_OR_source_OR_stream));
     }
-    if ((blob_OR_source_OR_stream is MediaStream)) {
+    if ((blob_OR_source_OR_stream is MediaSource)) {
       return _blink.BlinkURL.instance.createObjectURL_Callback_1_(unwrap_jso(blob_OR_source_OR_stream));
     }
-    if ((blob_OR_source_OR_stream is MediaSource)) {
+    if ((blob_OR_source_OR_stream is MediaStream)) {
       return _blink.BlinkURL.instance.createObjectURL_Callback_1_(unwrap_jso(blob_OR_source_OR_stream));
     }
     throw new ArgumentError("Incorrect number or type of arguments");