✨ [Observable] Make observable safe to remove handler while firing.

src/core/data-structures/observable.js

@@ -58,7 +58,8 @@ export class Observable {
     if (!this.handlers_) {
       return;
     }
-    for (const handler of this.handlers_) {
+    // Iterate over copy of handlers_ in case handlers are removed inside.
+    for (const handler of this.handlers_.slice()) {
       handler(opt_event);
     }
   }

test/unit/core/data-structures/test-observable.js

@@ -44,4 +44,20 @@ describes.sandboxed('data structures - Observable', {}, () => {
     expect(observer1Called).to.equal(1);
     expect(observer2Called).to.equal(2);
   });
+
+  it('remove while firing', () => {
+    let observer1Called = 0;
+    const observer1 = () => {
+      observer1Called++;
+    };
+    observable.add(observer1);
+    const remove = observable.add(() => {
+      remove();
+    });
+    observable.add(observer1);
+
+    observable.fire();
+
+    expect(observer1Called).to.equal(2);
+  });
 });