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

test/codegen/expect/opassign.js

+var opassign;
+(function(exports) {
+  'use strict';
+  dart.copyProperties(exports, {
+    get index() {
+      core.print('called "index" getter');
+      return 0;
+    }
+  });
+  dart.defineLazyProperties(exports, {
+    get _foo() {
+      return new Foo();
+    }
+  });
+  dart.copyProperties(exports, {
+    get foo() {
+      core.print('called "foo" getter');
+      return exports._foo;
+    }
+  });
+  class Foo extends core.Object {
+    Foo() {
+      this.x = 100;
+    }
+  }
+  // Function main: () → dynamic
+  function main() {
+    let f = dart.map([0, 40]);
+    core.print('should only call "index" 2 times:');
+    let i = exports.index;
+    f.set(i, dart.dbinary(f.get(i), '+', 1));
+    forcePostfix((() => {
+      let i = exports.index, x$ = f.get(i);
+      f.set(i, dart.dbinary(x$, '+', 1));
+      return x$;
+    })());
+    core.print('should only call "foo" 2 times:');
+    let o = exports.foo;
+    dart.dput(o, 'x', dart.dbinary(dart.dload(o, 'x'), '+', 1));
+    forcePostfix((() => {
+      let o = exports.foo, x$ = dart.dload(o, 'x');
+      dart.dput(o, 'x', dart.dbinary(x$, '+', 1));
+      return x$;
+    })());
+    core.print('op assign test, should only call "index" twice:');
+    let i = exports.index;
+    f.set(i, dart.dbinary(f.get(i), '+', f.get(exports.index)));
+  }
+  // Function forcePostfix: (dynamic) → dynamic
+  function forcePostfix(x) {
+  }
+  // Exports:
+  exports.Foo = Foo;
+  exports.main = main;
+  exports.forcePostfix = forcePostfix;
+})(opassign || (opassign = {}));