implement opassign, fix bugs in pre/postfix, introduce a let* helper

test/codegen/expect/cascade.js

 var cascade;
 (function(exports) {
   'use strict';
   class A extends core.Object {
     A() {
       this.x = null;
     }
   }
   // Function test_closure_with_mutate: () → void
   function test_closure_with_mutate() {
     let a = new A();
     a.x = () => {
       core.print("hi");
       a = null;
     };
-    ((_) => {
-      dart.dinvoke(_, 'x');
-      dart.dinvoke(_, 'x');
-    })(a);
+    let _ = a;
+    dart.dinvoke(_, 'x');
+    dart.dinvoke(_, 'x');
     core.print(a);
   }
   // Function test_closure_without_mutate: () → void
   function test_closure_without_mutate() {
     let a = new A();
     a.x = () => {
       core.print(a);
     };
     dart.dinvoke(a, 'x');
     dart.dinvoke(a, 'x');
     core.print(a);
   }
   // Function test_mutate_inside_cascade: () → void
   function test_mutate_inside_cascade() {
     let a = null;
-    a = ((_) => {
-      _.x = a = null;
-      _.x = a = null;
-      return _;
-    })(new A());
+    let _ = new A();
+    _.x = a = null;
+    _.x = a = null;
+    a = _;
     core.print(a);
   }
   // Function test_mutate_outside_cascade: () → void
   function test_mutate_outside_cascade() {
     let a = null, b = null;
     a = new A();
-    dart.dput(a, 'x', b = null);
-    dart.dput(a, 'x', b = null);
+    a.x = b = null;
+    a.x = b = null;
     a = null;
     core.print(a);
   }
   // Function test_VariableDeclaration_single: () → void
   function test_VariableDeclaration_single() {
     let a = new core.List.from([]);
-    dart.dput(a, 'length', 2);
-    a.add(42);
+    a[core.$length] = 2;
+    a[core.$add](42);
     core.print(a);
   }
   // Function test_VariableDeclaration_last: () → void
   function test_VariableDeclaration_last() {
-    let a = 42, b = new core.List.from([]);
-    dart.dput(b, 'length', 2);
-    b.add(a);
+    let a = 42, b = (() => {
+      let _ = new core.List.from([]);
+      _[core.$length] = 2;
+      _[core.$add](a);
+      return _;
+    })();
     core.print(b);
   }
   // Function test_VariableDeclaration_first: () → void
   function test_VariableDeclaration_first() {
-    let a = ((_) => {
+    let a = (() => {
+      let _ = new core.List.from([]);
       _[core.$length] = 2;
       _[core.$add](3);
       return _;
-    })(new core.List.from([])), b = 2;
+    })(), b = 2;
     core.print(a);
   }
   let Base$ = dart.generic(function(T) {
     class Base extends core.Object {
       Base() {
         this.x = new core.List$(T).from([]);
       }
     }
     return Base;
   });
@@ skipping 12 matching lines @@
   exports.test_closure_without_mutate = test_closure_without_mutate;
   exports.test_mutate_inside_cascade = test_mutate_inside_cascade;
   exports.test_mutate_outside_cascade = test_mutate_outside_cascade;
   exports.test_VariableDeclaration_single = test_VariableDeclaration_single;
   exports.test_VariableDeclaration_last = test_VariableDeclaration_last;
   exports.test_VariableDeclaration_first = test_VariableDeclaration_first;
   exports.Base$ = Base$;
   exports.Base = Base;
   exports.Foo = Foo;
 })(cascade || (cascade = {}));