Feature-detect browser for assignment to __proto__.

pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart

Index: pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart b/pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart
index 5bd1cb1d685faabde990fcdece9ab37e4ed9528a..6301e77d964edfdc6892b7164203d504f29968fb 100644
--- a/pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/full_emitter/emitter.dart
@@ -1067,9 +1067,28 @@ class Emitter implements js_emitter.Emitter {
           var cls = function () {};
           cls.prototype = {'p': {}};
           var object = new cls();
-          return object.__proto__ &&
-                 object.__proto__.p === cls.prototype.p;
-         })();
+          if (!(object.__proto__ && object.__proto__.p === cls.prototype.p))
+            return false;
+    
+          try {
+            // Are we running on a platform where the performance is good?
+            // (i.e. Chrome or d8).
+
+            // Chrome userAgent?
+            if (typeof navigator != "undefined" &&
+                typeof navigator.userAgent == "string" &&
+                navigator.userAgent.indexOf("Chrome/") >= 0) return true;
+
+            // d8 version() looks like "N.N.N.N", jsshell version() like "N".
+            if (typeof version == "function" &&
+                version.length == 0) {
+              var v = version();
+              if (/^\d+\.\d+\.\d+\.\d+$/.test(v)) return true;
+            }
+          } catch(_) {}
+          
+          return false;
+        })();
       ''');
     }