fix(compiler): fix a few non-tree-shakeable code patterns (angular#24677)

This change makes @angular/compiler more tree-shakeable by changing
an enum to a const enum and by getting rid of a top-level map that
the tree-shaker was seeing as a reference which caused r3_identifiers
to be retained.

This drops a few hundred bytes of JS from tree-shaken ngtsc compiled
apps.

PR Close angular#24677

Author: alxhub

packages/compiler/src/render3/view/template.ts

@@ -32,12 +32,20 @@ import {htmlAstToRender3Ast} from '../r3_template_transform';
 import {R3QueryMetadata} from './api';
 import {CONTEXT_NAME, I18N_ATTR, I18N_ATTR_PREFIX, ID_SEPARATOR, IMPLICIT_REFERENCE, MEANING_SEPARATOR, REFERENCE_PREFIX, RENDER_FLAGS, TEMPORARY_NAME, asLiteral, getQueryPredicate, invalid, mapToExpression, noop, temporaryAllocator, trimTrailingNulls, unsupported} from './util';
 
-const BINDING_INSTRUCTION_MAP: {[type: number]: o.ExternalReference} = {
-  [BindingType.Property]: R3.elementProperty,
-  [BindingType.Attribute]: R3.elementAttribute,
-  [BindingType.Class]: R3.elementClassNamed,
-  [BindingType.Style]: R3.elementStyleNamed,
-};
+function mapBindingToInstruction(type: BindingType): o.ExternalReference|undefined {
+  switch (type) {
+    case BindingType.Property:
+      return R3.elementProperty;
+    case BindingType.Attribute:
+      return R3.elementAttribute;
+    case BindingType.Class:
+      return R3.elementClassNamed;
+    case BindingType.Style:
+      return R3.elementStyleNamed;
+    default:
+      return undefined;
+  }
+}
 
 // `className` is used below instead of `class` because the interception
 // code (where this map is used) deals with DOM element property values
@@ -414,7 +422,7 @@ export class TemplateDefinitionBuilder implements t.Visitor<void>, LocalResolver
         return;
       }
 
-      const instruction = BINDING_INSTRUCTION_MAP[input.type];
+      const instruction = mapBindingToInstruction(input.type);
       if (instruction) {
         // TODO(chuckj): runtime: security context?
         this.instruction(

packages/compiler/src/template_parser/template_ast.ts

@@ -58,7 +58,7 @@ export class AttrAst implements TemplateAst {
   visit(visitor: TemplateAstVisitor, context: any): any { return visitor.visitAttr(this, context); }
 }
 
-export enum PropertyBindingType {
+export const enum PropertyBindingType {
   // A normal binding to a property (e.g. `[property]="expression"`).
   Property,
   // A binding to an element attribute (e.g. `[attr.name]="expression"`).

packages/core/test/bundling/hello_world_r2/bundle.golden_symbols.json

@@ -356,9 +356,6 @@
   {
     "name": "CORE"
   },
-  {
-    "name": "CORE$1"
-  },
   {
     "name": "CR"
   },
@@ -803,9 +800,6 @@
   {
     "name": "Identifiers"
   },
-  {
-    "name": "Identifiers$1"
-  },
   {
     "name": "IfStmt"
   },
@@ -1208,9 +1202,6 @@
   {
     "name": "PreparsedElementType"
   },
-  {
-    "name": "PropertyBindingType"
-  },
   {
     "name": "PropertyRead"
   },