Fix duplicate context creation when closures appear in initializers.

pkg/kernel/testcases/closures/closure_in_initializer_closure.dart.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class C extends core::Object {

[Dmitry Stefantsov, 2017/07/28 12:36:16]

I believe, this is the corresponding Dart code:

```
  C.foo(f)
      : t = (() {
          var prefix;
          var g = (x) {
            f("$prefix$x");
          };
          prefix = 'hest';
          return g;
        }) {
    print(1);
  }
```

Here the captured variable is not used in the body of the constructor, and
there's no need for the redirecting technique here.

[sjindel, 2017/07/31 12:06:21]

Done.

   field dynamic t;
   constructor foo(dynamic f) → void
-    : final Vector #context = MakeVector(2), dynamic #t1 = #context[1] = f, self::C::t = MakeClosure<() → dynamic>(self::closure#C#foo#function, #context), super core::Object::•() {
-    final Vector #context = MakeVector(2);
-    #context[1] = f;
+    : final Vector #context = MakeVector(2), this self::C::foo#redir(f, #context)
+    ;
+  constructor foo#redir(dynamic f, final Vector #context) → void
+    : dynamic #t1 = #context[1] = f, self::C::t = MakeClosure<() → dynamic>(self::closure#C#foo#function, #context), super core::Object::•() {
     core::print(1);
   }
 }
 static method main() → dynamic {
   core::print(0);
   dynamic c = new self::C::foo(MakeClosure<(dynamic) → dynamic>(self::closure#main#function, null));
   core::print(2);
   c.t().call("fisk");
   core::print(3);
 }
 static method closure#C#foo#function#function(Vector #contextParameter, dynamic x) → dynamic {
   (#contextParameter[0][1]).call("${#contextParameter[1]}${x}");
 }
 static method closure#C#foo#function(Vector #contextParameter) → dynamic {
   final Vector #context = MakeVector(2);
   #context[0] = #contextParameter;
   #context[1] = null;
   dynamic g = MakeClosure<(dynamic) → dynamic>(self::closure#C#foo#function#function, #context);
   #context[1] = "hest";
   return g;
 }
 static method closure#main#function(Vector #contextParameter, dynamic x) → dynamic {
   return core::print(x);
 }