Incorrect "JSC_USED_GLOBAL_THIS" warning on `this` in class field initializer

[lauraharker]

As noted in #2731 - the compiler incorrectly reports "[JSC_USED_GLOBAL_THIS] dangerous use of the global this object" on the this.y reference below.

class Foo {
  x = () => this.y;
  y = 3;
}

this in a non-static class field initializer will refer to the class instance.

To reproduce -

closure-compiler-debugger

[lauraharker]

This also repros when this is not wrapped in an arrow function.