Optimize ES2015 static fields

[filipesilva]

🚀 Feature request

Description

Using the ES2015 target, TypeScript transpiles classes with static fields as a class plus the assignment of the static fields over the class itself.

As an example, the TemplateRef class in https://github.com/angular/angular/blob/73dcd72afbd782161c1674f1e647a1b357a78a67/packages/core/src/linker/template_ref.ts#L34-L62:

export abstract class TemplateRef<C> {
  abstract get elementRef(): ElementRef;

  abstract createEmbeddedView(context: C): EmbeddedViewRef<C>;

  static __NG_ELEMENT_ID__:
      () => TemplateRef<any>| null = () => SWITCH_TEMPLATE_REF_FACTORY(TemplateRef, ElementRef)
}

Is transpiled to ES2015 as:

class TemplateRef {
}
TemplateRef.__NG_ELEMENT_ID__ = () => SWITCH_TEMPLATE_REF_FACTORY(TemplateRef, ElementRef);

The property assignment in the transpiled code prevents the class from being removed by minifiers such as terser.

Describe the solution you'd like

Build Optimizer converts ES2015 static fields in such a way that they are not retained after minification.

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