Rename ClosureVariable, use separate IR forms for declaration and assignment.

pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart

Index: pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
diff --git a/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart b/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
index 6ea807b18b3694a6199f4c53deaa975ac95e51a5..1921450f6d660e3215f4a8ac032bee81daf287f2 100644
--- a/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
+++ b/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
@@ -492,9 +492,10 @@ class Assign extends Statement {
   Variable variable;
   Expression definition;
 
-  /// If true, this declares a new copy of the closure variable.
-  /// The consequences are similar to [cps_ir.SetClosureVariable].
-  /// All uses of the variable must be nested inside the [next] statement.
+  /// If true, this assignes to a fresh variable scoped to the [next]
+  /// statement.
+  ///
+  /// Variable declarations themselves are hoisted to function level.
   bool isDeclaration;
 
   Assign(this.variable, this.definition, this.next,