Fix duplicate context creation when closures appear in initializers.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 class C1 extends core::Object {
   field dynamic x;
   constructor •(dynamic y) → void
     : final Vector #context = MakeVector(2), dynamic #t1 = #context[1] = y, self::C1::x = MakeClosure<() → dynamic>(self::closure#C1#function#function, #context), super core::Object::•()
     ;
 }
 class C2 extends core::Object {

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

If I'm getting this right, here is the Dart code for this class:

```
class C2 {
  var x;
  C2(y) {
    x = () => print('Hello $y');
  }
}
```

My understanding is that we shouldn't apply this technique with redirecting
constructors here, because there are no initializers.

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

Done.

   field dynamic x = null;
   constructor •(dynamic y) → void
-    : final Vector #context = MakeVector(2), dynamic #t2 = #context[1] = y, super core::Object::•() {
-    final Vector #context = MakeVector(2);
-    #context[1] = y;
+    : final Vector #context = MakeVector(2), this self::C2::#redir(y, #context)
+    ;
+  constructor #redir(dynamic y, final Vector #context) → void
+    : dynamic #t2 = #context[1] = y, super core::Object::•() {
     this.x = MakeClosure<() → dynamic>(self::closure#C2#function#function, #context);
   }
 }
 static method main() → dynamic {
   new self::C1::•("hest").x();
   new self::C2::•("naebdyr").x();
 }
 static method closure#C1#function#function(Vector #contextParameter) → dynamic {
   return core::print("Hello ${#contextParameter[1]}");
 }
 static method closure#C2#function#function(Vector #contextParameter) → dynamic {
   return core::print("Hello ${#contextParameter[1]}");
 }