use VariableElement instead of VariableElementImpl in _isStateless

lib/src/codegen/js_codegen.dart

Index: lib/src/codegen/js_codegen.dart
diff --git a/lib/src/codegen/js_codegen.dart b/lib/src/codegen/js_codegen.dart
index 276fa620f2dec7daff5669589b421c9845083e80..8377913cf1938d853e8cf556953e8f5c2dd86906 100644
--- a/lib/src/codegen/js_codegen.dart
+++ b/lib/src/codegen/js_codegen.dart
@@ -1697,7 +1697,7 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
     if (node is SimpleIdentifier) {
       var e = node.staticElement;
       if (e is PropertyAccessorElement) e = e.variable;
-      if (e is VariableElementImpl && !e.isSynthetic) {
+      if (e is VariableElement && !e.isSynthetic) {

[Brian Wilkerson, 2015/04/04 15:58:24]

This is a good first step, but doesn't fix the problem. If 'e' is an instance of
ParameterMember it will pass this 'is' check, but will fail the 'is' check below
(line 1702).

We really need to address this issue in the Element API, but one ugly work
around would be to add the following before line 1702:
  // TODO Remove the following statement and convert the condition below to use
the public classes
  // once _isPotentiallyMutated does not require an implementation class.
  if (e is Member) {
    e = e.baseElement;
  }

[Jennifer Messerly, 2015/04/06 15:19:22]

Sounds good, but how can I get a ParameterMember here to test it? This code is
used when compiling a Dart method cascade, such as:

    x..add(1)..add(2)

How can I make "x" be a ParameterMember? I tried:

class Generic<T extends List<int>> {
  test_generic_type_parameter(T x) {
    x..add(1)..add(2)..add(3)..add(4);
  }
}

class Derived extends Generic<List<int>> {
  test_generic_type_parameter(List<int> x) {
    x..add(1)..add(2)..add(3)..add(4);
  }
}

Neither case was able to cause a problem.

[Brian Wilkerson, 2015/04/06 15:23:05]

I don't think you can. My comment was based on a misunderstanding on my part of
how this code was being used, and should be ignored. It's fine the way it is.

[Jennifer Messerly, 2015/04/06 15:46:31]

Ah that's good. I think I still like your variation `if (e is Member) e =
e.baseMember;` better as it seems cleaner and that way if someone later reused
this code, we'll be safer :). I'll also make the comment clear that this
function (_isStateless) is intended to be conservative, and should not be relied
on to correctly detect all cases.

         if (e.isFinal) return true;
         if (e is LocalVariableElementImpl || e is ParameterElementImpl) {
           // make sure the local isn't mutated in the context.