[BUGFIX] Use WeakMap to store tracked property values.

Using a `Symbol` causes issues on IE11 when using the core-js polyfill.
Specifically, the polyfill adds every symbol to `Object.prototype` and
therefore our `if (symbol in this) {` check was completely borked.

Migrating to a `WeakMap` exposed another issue: we were accidentally
relying on the fact that the symbols would be inherited through the
prototype chain. This meant that setting a tracked property on a
prototype would actually affect the value seen by every instance of that
prototype.

For example:

```js
class Thing {
  @Tracked baz;
}

Thing.prototype.baz = 'foo';

let thing = new Thing();

console.log(thing.baz)
```

Before the changes in this commit, the above console.log would emit
`foo` and after these changes it would print `undefined`.

packages/@ember/-internals/metal/lib/tracked.ts

@@ -1,4 +1,4 @@
-import { HAS_NATIVE_SYMBOL, isEmberArray, symbol as emberSymbol } from '@ember/-internals/utils';
+import { isEmberArray } from '@ember/-internals/utils';
 import { EMBER_NATIVE_DECORATOR_SUPPORT } from '@ember/canary-features';
 import { assert } from '@ember/debug';
 import { DEBUG } from '@glimmer/env';
@@ -9,10 +9,6 @@ import { markObjectAsDirty, tagForProperty, update } from './tags';
 
 type Option<T> = T | null;
 
-// For some reason TS can't infer that these two functions are compatible-ish,
-// so we need to corece the type
-let symbol = (HAS_NATIVE_SYMBOL ? Symbol : emberSymbol) as (debugKey: string) => string;
-
 /**
   An object that that tracks @tracked properties that were consumed.
 
@@ -192,7 +188,8 @@ function descriptorForField([_target, key, desc]: [
   );
 
   let initializer = desc ? desc.initializer : undefined;
-  let secretKey = symbol(key);
+  let values = new WeakMap();
+  let hasInitializer = typeof initializer === 'function';
 
   return {
     enumerable: true,
@@ -203,26 +200,30 @@ function descriptorForField([_target, key, desc]: [
 
       if (CURRENT_TRACKER) CURRENT_TRACKER.add(propertyTag);
 
+      let value;
+
       // If the field has never been initialized, we should initialize it
-      if (!(secretKey in this)) {
-        this[secretKey] = typeof initializer === 'function' ? initializer.call(this) : undefined;
-      }
+      if (hasInitializer && !values.has(this)) {
+        value = initializer.call(this);
 
-      let value = this[secretKey];
+        values.set(this, value);
+      } else {
+        value = values.get(this);
+      }
 
       // Add the tag of the returned value if it is an array, since arrays
       // should always cause updates if they are consumed and then changed
       if (Array.isArray(value) || isEmberArray(value)) {
         update(propertyTag, tagForProperty(value, '[]'));
       }
 
-      return this[secretKey];
+      return value;
     },
 
     set(newValue: any): void {
       markObjectAsDirty(this, key);
 
-      this[secretKey] = newValue;
+      values.set(this, newValue);
 
       if (propertyDidChange !== null) {
         propertyDidChange();

packages/@ember/controller/tests/controller_test.js

@@ -138,10 +138,10 @@ moduleFor(
       assert.expect(3);
       expectNoDeprecation();
 
-      let controller = Controller.extend({
+      let controller = Controller.create({
         content: 'foo-bar',
         model: 'blammo',
-      }).create();
+      });
 
       assert.equal(get(controller, 'content'), 'foo-bar');
       assert.equal(get(controller, 'model'), 'blammo');

packages/ember/tests/routing/decoupled_basic_test.js

@@ -155,8 +155,9 @@ moduleFor(
       this.add(
         'controller:homepage',
         Controller.extend({
-          model: {
-            home: 'Comes from homepage',
+          init() {
+            this._super(...arguments);
+            this.model = { home: 'Comes from homepage' };
           },
         })
       );
@@ -183,16 +184,18 @@ moduleFor(
       this.add(
         'controller:foo',
         Controller.extend({
-          model: {
-            home: 'Comes from foo',
+          init() {
+            this._super(...arguments);
+            this.model = { home: 'Comes from foo' };
           },
         })
       );
       this.add(
         'controller:homepage',
         Controller.extend({
-          model: {
-            home: 'Comes from homepage',
+          init() {
+            this._super(...arguments);
+            this.model = { home: 'Comes from homepage' };
           },
         })
       );
@@ -220,8 +223,9 @@ moduleFor(
       this.add(
         'controller:home',
         Controller.extend({
-          model: {
-            home: 'Comes from home.',
+          init() {
+            this._super(...arguments);
+            this.model = { home: 'Comes from home.' };
           },
         })
       );
@@ -241,8 +245,9 @@ moduleFor(
       this.add(
         'controller:home',
         Controller.extend({
-          model: {
-            home: 'YES I AM HOME',
+          init() {
+            this._super(...arguments);
+            this.model = { home: 'YES I AM HOME' };
           },
         })
       );