[dart2js] Never elide _Enum.index

This ensures that `_Enum.index` is always available for switch strength
reduction optimizations. Most real programs use the index of at least one `enum`, so the field is not usually elided. This change makes small tests and benchmarks behave more like real programs where these optimizations happen.

Bug: #51657
CoreLibraryReviewExempt: dart2js specific annotation
Change-Id: If98e483d7cce265e823fd574565089e328215cca
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/446481
Reviewed-by: Nate Biggs <natebiggs@google.com>
Commit-Queue: Stephen Adams <sra@google.com>
Reviewed-by: Mayank Patke <fishythefish@google.com>

benchmarks/SwitchFSM/dart/SwitchFSM.dart

@@ -136,12 +136,6 @@ class Benchmark extends BenchmarkBase {
 enum SomeEnum { element }
 
 void main() {
-  // TODO(http://dartbug.com/51657): dart2js will remove `_Enum.index` in simple
-  // programs that don't appear to use the field. This defeats the enum-switch
-  // optimization that works more reliably in larger programs. Remove this code
-  // that marks `_Enum.index` as used when #51657 is fixed.
-  Expect.equals(0, SomeEnum.element.index);
-
   final benchmarks = [
     Benchmark('enum', match_enum.match),
     Benchmark('int', match_int.match),

pkg/compiler/test/codegen/model_data/effectively_constant_state.dart

@@ -4,16 +4,15 @@
 
 enum Enum { a, b, c }
 
-/*member: tester1:params=0*/
-@pragma('dart2js:noInline')
+@pragma('dart2js:never-inline')
 tester1() {}
 
 /*member: tester2:params=0*/
-@pragma('dart2js:noInline')
+@pragma('dart2js:never-inline')
 tester2() {}
 
 /*member: tester3:params=0*/
-@pragma('dart2js:noInline')
+@pragma('dart2js:never-inline')
 tester3() {}
 
 class Class {
@@ -26,7 +25,7 @@ class Class {
   Class({this.state1 = 1, this.state2 = Enum.c});
 
   /*member: Class.method1a:calls=[tester2(0)],params=0*/
-  @pragma('dart2js:noInline')
+  @pragma('dart2js:never-inline')
   method1a() {
     if (state1 == 0) {
       return tester1();
@@ -39,7 +38,7 @@ class Class {
 
   // TODO(johnniwinther): Inline switch cases with constant expressions.
   /*member: Class.method1b:calls=[tester2(0)],params=0,switch*/
-  @pragma('dart2js:noInline')
+  @pragma('dart2js:never-inline')
   method1b() {
     switch (state1) {
       case 0:
@@ -52,7 +51,7 @@ class Class {
   }
 
   /*member: Class.method2a:calls=[tester3(0)],params=0*/
-  @pragma('dart2js:noInline')
+  @pragma('dart2js:never-inline')
   method2a() {
     if (state2 == Enum.a) {
       return tester1();
@@ -63,10 +62,9 @@ class Class {
     }
   }
 
-  /*member: Class.method2b:calls=[tester1(0),tester2(0),tester3(0)],params=0,switch*/
-  @pragma('dart2js:noInline')
+  /*member: Class.method2b:calls=[tester3(0)],params=0,switch*/
+  @pragma('dart2js:never-inline')
   method2b() {
-    // TODO(johnniwinther): Eliminate dead code in enum switch.
     switch (state2) {
       case Enum.a:
         return tester1();

sdk/lib/core/enum.dart

@@ -102,6 +102,8 @@ abstract interface class Enum {
 /// Superclass of all enum class implementations.
 @pragma('dyn-module:language-impl:extendable')
 abstract class _Enum implements Enum {
+  // See http://dartbug.com/51657 for discussion of dart2js pragma.
+  @pragma('dart2js:noElision')
   final int index;
 
   @pragma('dyn-module:language-impl:callable')