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

lib/runtime/dart/_interceptors.js

 var _interceptors;
 (function(exports) {
   'use strict';
   let JSMutableArray$ = dart.generic(function(E) {
     class JSMutableArray extends JSArray$(E) {}
     JSMutableArray[dart.implements] = () => [JSMutableIndexable];
     return JSMutableArray;
   });
   let JSMutableArray = JSMutableArray$();
   let JSFixedArray$ = dart.generic(function(E) {
@@ skipping 193 matching lines @@
     }
     static [_handleIEtoString](result) {
       let match = /^([\da-z]+)(?:\.([\da-z]+))?\(e\+(\d+)\)$/.exec(result);
       if (match == null) {
         throw new core.UnsupportedError(`Unexpected toString result: ${result}`);
       }
       let result = dart.dindex(match, 1);
       let exponent = +dart.dindex(match, 3);
       if (dart.dindex(match, 2) != null) {
         result = result + dart.dindex(match, 2);
-        exponent = dart.dindex(match, 2).length;
+        exponent = dart.notNull(exponent) - dart.dindex(match, 2).length;
       }
       return core.String['+'](result, core.String['*']("0", exponent));
     }
     toString() {
       if (dart.notNull(dart.equals(this, 0)) && 1 / this < 0) {
         return '-0.0';
       } else {
         return "" + this;
       }
     }
@@ skipping 156 matching lines @@
       if (dart.notNull(nonneg) >= 4294967296) {
         nonneg = (dart.notNull(nonneg) / 4294967296).truncate();
         return dart.notNull(JSInt[_bitCount](JSInt[_spread](nonneg))) + 32;
       }
       return JSInt[_bitCount](JSInt[_spread](nonneg));
     }
     static [_bitCount](i) {
       i = dart.as(dart.dbinary(JSInt[_shru](i, 0), '-', dart.dbinary(JSInt[_shru](i, 1), '&', 1431655765)), core.int);
       i = (dart.notNull(i) & 858993459)['+'](dart.dbinary(JSInt[_shru](i, 2), '&', 858993459));
       i = 252645135 & i['+'](JSInt[_shru](i, 4));
-      i = dart.as(JSInt[_shru](i, 8), core.int);
-      i = dart.as(JSInt[_shru](i, 16), core.int);
+      i = dart.notNull(i) + dart.notNull(dart.as(JSInt[_shru](i, 8), core.int));
+      i = dart.notNull(i) + dart.notNull(dart.as(JSInt[_shru](i, 16), core.int));
       return dart.notNull(i) & 63;
     }
     static [_shru](value, shift) {
       return value >>> shift;
     }
     static [_shrs](value, shift) {
       return value >> shift;
     }
     static [_ors](a, b) {
       return a | b;
@@ skipping 319 matching lines @@
         throw new core.OutOfMemoryError();
       }
       let result = '';
       let s = this;
       while (true) {
         if ((dart.notNull(times) & 1) == 1)
           result = s['+'](result);
         times = dart.as(times >>> 1, core.int);
         if (times == 0)
           break;
-        s = s;
+        s = s['+'](s);
       }
       return result;
     }
     padLeft(width, padding) {
       if (padding === void 0)
         padding = ' ';
       let delta = dart.notNull(width) - dart.notNull(this.length);
       if (dart.notNull(delta) <= 0)
         return this;
       return core.String['+'](core.String['*'](padding, delta), this);
@@ skipping 206 matching lines @@
   dart.copyProperties(exports, {
     get mapTypeToInterceptor() {
       return _foreign_helper.JS_EMBEDDED_GLOBAL('', _js_embedded_names.MAP_TYPE_TO_INTERCEPTOR);
     }
   });
   // Function findIndexForNativeSubclassType: (Type) → int
   function findIndexForNativeSubclassType(type) {
     if (exports.mapTypeToInterceptor == null)
       return null;
     let map = dart.as(exports.mapTypeToInterceptor, core.List);
-    for (let i = 0; dart.notNull(i) + 1 < dart.notNull(map[core.$length]); i = 3) {
+    for (let i = 0; dart.notNull(i) + 1 < dart.notNull(map[core.$length]); i = dart.notNull(i) + 3) {
       if (dart.equals(type, map[core.$get](i))) {
         return i;
       }
     }
     return null;
   }
   // Function findInterceptorConstructorForType: (Type) → dynamic
   function findInterceptorConstructorForType(type) {
     let index = findIndexForNativeSubclassType(type);
     if (index == null)
@@ skipping 112 matching lines @@
   exports.findInterceptorForType = findInterceptorForType;
   exports.JSBool = JSBool;
   exports.JSNull = JSNull;
   exports.JSIndexable = JSIndexable;
   exports.JSMutableIndexable = JSMutableIndexable;
   exports.JSObject = JSObject;
   exports.JavaScriptObject = JavaScriptObject;
   exports.PlainJavaScriptObject = PlainJavaScriptObject;
   exports.UnknownJavaScriptObject = UnknownJavaScriptObject;
 })(_interceptors || (_interceptors = {}));