Remove factory body in *.fromEnvironment, and implement this same behavior

pkg/compiler/lib/src/js_backend/backend_impact.dart

Index: pkg/compiler/lib/src/js_backend/backend_impact.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend_impact.dart b/pkg/compiler/lib/src/js_backend/backend_impact.dart
index 14aaca53e56b459ab7a6ea100351ac74bf8de13e..52f371ecea4df316e6790153099766cd59694a94 100644
--- a/pkg/compiler/lib/src/js_backend/backend_impact.dart
+++ b/pkg/compiler/lib/src/js_backend/backend_impact.dart
@@ -303,6 +303,17 @@ class BackendImpacts {
         ]);
   }
 
+  BackendImpact _throwUnsupportedError;
+
+  BackendImpact get throwUnsupportedError {
+    return _throwUnsupportedError ??= new BackendImpact(staticUses: [
+      _commonElements.throwUnsupportedError
+    ], otherImpacts: [
+      // Also register the types of the arguments passed to this method.
+      stringValues
+    ]);
+  }
+
   BackendImpact _superNoSuchMethod;
 
   BackendImpact get superNoSuchMethod {