Adding compound assignments to the top-level compiler.

src/ast.h

Index: src/ast.h
===================================================================
--- src/ast.h	(revision 3445)
+++ src/ast.h	(working copy)
@@ -1241,6 +1241,8 @@
   Expression* target() const { return target_; }
   Expression* value() const { return value_; }
   int position() { return pos_; }
+  // This check relies on the definition order of token in token.h.
+  bool is_compound() const { return op() > Token::ASSIGN; }
 
   // An initialization block is a series of statments of the form
   // x.y.z.a = ...; x.y.z.b = ...; etc. The parser marks the beginning and