Address late comments from CL 50313007.

dart/sdk/lib/_internal/lib/js_helper.dart

Index: dart/sdk/lib/_internal/lib/js_helper.dart
diff --git a/dart/sdk/lib/_internal/lib/js_helper.dart b/dart/sdk/lib/_internal/lib/js_helper.dart
index ba5cf5bf68c5ba31092a8d792d645a91e63fd818..2837596419a8c53f56a84724dce8f6dfb664836c 100644
--- a/dart/sdk/lib/_internal/lib/js_helper.dart
+++ b/dart/sdk/lib/_internal/lib/js_helper.dart
@@ -258,6 +258,8 @@ class CachedInvocation {
   }
 }
 
+/// A "catch all" invocation is when we cannot directly call a stub. Instead, we
+/// must extend the provided arguments with default arguments.
 class CachedCatchAllInvocation extends CachedInvocation {
   final ReflectionInfo info;
 
@@ -1785,7 +1787,7 @@ abstract class Closure implements Function {
 
     var functionType = info.functionType;
 
-    // Create a closure and "monkey" patch it with call stubs.
+    // Create a closure.
     Closure closure;
     var trampoline = function;
     if (!isStatic) {
@@ -1819,6 +1821,7 @@ abstract class Closure implements Function {
 
     JS('', '#.\$signature = #', closure, signatureFunction);
 
+    // "Monkey" patch the closure with call stubs.
     JS('', '#[#] = #', closure, callName, trampoline);
     for (int i = 1; i < functions.length; i++) {
       var stub = functions[i];
@@ -1828,6 +1831,7 @@ abstract class Closure implements Function {
          isStatic ? stub : forwardTo(receiver, stub));
     }
 
+    // Add "catch-all" stub. See [CachedCatchAllInvocation].
     JS('', '#["call*"] = #', closure, function);
 
     return closure;
@@ -2373,6 +2377,9 @@ abstract class RuntimeType {
   toRti();
 }
 
+/// Tell type inferrer it can't infer the type of [expression].
+obfuscate(expression) => JS('', '#', expression);
+
 class RuntimeFunctionType extends RuntimeType {
   final RuntimeType returnType;
   final List<RuntimeType> parameterTypes;
@@ -2399,18 +2406,14 @@ class RuntimeFunctionType extends RuntimeType {
   }
 
   @NoInline() @NoSideEffects()
-  _asCheck(expression) {
-    // Type inferrer doesn't think this is called with dynamic arguments.
-    return _check(JS('', '#', expression), true);
-  }
+  _asCheck(expression) => _check(obfuscate(expression), true);
 
   @NoInline() @NoSideEffects()
   _assertCheck(expression) {
     if (inAssert) return;
     inAssert = true; // Don't try to check this library itself.
     try {
-      // Type inferrer don't think this is called with dynamic arguments.
-      return _check(JS('', '#', expression), false);
+      return _check(obfuscate(expression), false);
     } finally {
       inAssert = false;
     }