[dart2js/ddc] Make `Object` members consistent for JS objects

[srujzs]

Currently there are the following discrepancies between the two compilers for native and interop objects:

• DDC returns a random hash code for interop objects, while dart2js returns 0. Both of these need to be updated to use a ES6 WeakMap to correctly give hashes for these objects.
• dart2js returns Instance of '<specific type>' for toString while DDC forwards to the underlying toString of the JS object. dart2js should use the DDC model here.
• DDC returns LegacyJavaScriptObject for runtimeType on interop objects, while dart2js returns JSObject. It's unclear what value we want here (perhaps JavaScriptObject?), but they should be consistent.