[dart2js] Enable CFE constant evaluation.

Change-Id: Ib449906bbfd95b6510e701606c43ef4c4169313b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/240601
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Joshua Litt <joshualitt@google.com>

pkg/compiler/lib/src/kernel/dart2js_target.dart

@@ -71,9 +71,12 @@ class Dart2jsTarget extends Target {
 
   final CompilerOptions? options;
   final bool canPerformGlobalTransforms;
+  final bool supportsUnevaluatedConstants;
 
   Dart2jsTarget(this.name, this.flags,
-      {this.options, this.canPerformGlobalTransforms = true});
+      {this.options,
+      this.canPerformGlobalTransforms = true,
+      this.supportsUnevaluatedConstants = true});
 
   @override
   bool get enableNoSuchMethodForwarders => true;
@@ -223,8 +226,8 @@ class Dart2jsTarget extends Target {
   }
 
   @override
-  ConstantsBackend get constantsBackend =>
+  ConstantsBackend get constantsBackend => Dart2jsConstantsBackend(
-      const Dart2jsConstantsBackend(supportsUnevaluatedConstants: true);
+      supportsUnevaluatedConstants: supportsUnevaluatedConstants);
 
   @override
   DartLibrarySupport get dartLibrarySupport =>

pkg/compiler/lib/src/options.dart

@@ -87,6 +87,9 @@ class FeatureOptions {
   /// to reduce generated code size.
   FeatureOption simpleAsyncToFuture = FeatureOption('simple-async-to-future');
 
+  /// Whether or not the CFE should evaluate constants.
+  FeatureOption cfeConstants = FeatureOption('cfe-constants');
+
   /// [FeatureOption]s which are shipped and cannot be toggled.
   late final List<FeatureOption> shipped = [
     newHolders,
@@ -102,7 +105,8 @@ class FeatureOptions {
   late final List<FeatureOption> canary = [
     writeUtf8,
     newDumpInfo,
-    simpleAsyncToFuture
+    simpleAsyncToFuture,
+    cfeConstants,
   ];
 
   /// Forces canary feature on. This must run after [Option].parse.

pkg/compiler/lib/src/phase/load_kernel.dart

@@ -213,8 +213,12 @@ Future<_LoadFromSourceResult> _loadFromSource(
     fe.InitializedCompilerState initializedCompilerState,
     String targetName) async {
   bool verbose = false;
+  bool cfeConstants = options.features.cfeConstants.isEnabled;
+  Map<String, String> environment = cfeConstants ? options.environment : null;
   Target target = Dart2jsTarget(targetName, TargetFlags(),
-      options: options, canPerformGlobalTransforms: true);
+      options: options,
+      canPerformGlobalTransforms: true,
+      supportsUnevaluatedConstants: !cfeConstants);
   fe.FileSystem fileSystem = CompilerFileSystem(compilerInput);
   fe.Verbosity verbosity = options.verbosity;
   fe.DiagnosticMessageHandler onDiagnostic = (fe.DiagnosticMessage message) {
@@ -237,6 +241,7 @@ Future<_LoadFromSourceResult> _loadFromSource(
       ..librariesSpecificationUri = options.librariesSpecificationUri
       ..packagesFileUri = options.packageConfig
       ..explicitExperimentalFlags = options.explicitExperimentalFlags
+      ..environmentDefines = environment
       ..verbose = verbose
       ..fileSystem = fileSystem
       ..onDiagnostic = onDiagnostic
@@ -266,6 +271,7 @@ Future<_LoadFromSourceResult> _loadFromSource(
       dependencies,
       options.packageConfig,
       explicitExperimentalFlags: options.explicitExperimentalFlags,
+      environmentDefines: environment,
       nnbdMode:
           options.useLegacySubtyping ? fe.NnbdMode.Weak : fe.NnbdMode.Strong,
       invocationModes: options.cfeInvocationModes,

pkg/front_end/lib/src/api_unstable/dart2js.dart

@@ -129,6 +129,7 @@ InitializedCompilerState initializeCompiler(
     List<Uri> additionalDills,
     Uri packagesFileUri,
     {required Map<ExperimentalFlag, bool> explicitExperimentalFlags,
+    Map<String, String>? environmentDefines,
     bool verify: false,
     NnbdMode? nnbdMode,
     Set<InvocationMode> invocationModes: const <InvocationMode>{},
@@ -144,6 +145,7 @@ InitializedCompilerState initializeCompiler(
       equalLists(oldState.options.additionalDills, additionalDills) &&
       equalMaps(oldState.options.explicitExperimentalFlags,
           explicitExperimentalFlags) &&
+      equalMaps(oldState.options.environmentDefines, environmentDefines) &&
       oldState.options.verify == verify &&
       oldState.options.nnbdMode == nnbdMode &&
       equalSets(oldState.options.invocationModes, invocationModes) &&
@@ -157,6 +159,8 @@ InitializedCompilerState initializeCompiler(
     ..librariesSpecificationUri = librariesSpecificationUri
     ..packagesFileUri = packagesFileUri
     ..explicitExperimentalFlags = explicitExperimentalFlags
+    ..environmentDefines = environmentDefines
+    ..errorOnUnevaluatedConstant = environmentDefines != null ? true : false
     ..verify = verify
     ..invocationModes = invocationModes
     ..verbosity = verbosity;

pkg/front_end/tool/_fasta/command_line.dart

@@ -234,6 +234,18 @@ ProcessedOptions analyzeCommandLine(String programName,
     });
   }
 
+  // In order to facilitate the roll out of CFE constants on Dart2js, we need to
+  // be able to support both passing '--no-defines' and also evaluating CFE
+  // constants when the target supports it. This is so we can easily enable or
+  // disable fully evaluating constants in the CFE with a flag. This can be
+  // deleted when the CFE fully evaluates constants for Dart2js in all cases.
+  Map<String, String>? environmentDefines;
+  if (noDefines && target.constantsBackend.supportsUnevaluatedConstants) {
+    // Pass a null environment.
+  } else {
+    environmentDefines = parsedOptions.defines;
+  }
+
   CompilerOptions compilerOptions = new CompilerOptions()
     ..compileSdk = compileSdk
     ..fileSystem = fileSystem
@@ -249,7 +261,7 @@ ProcessedOptions analyzeCommandLine(String programName,
     ..verify = verify
     ..skipPlatformVerification = skipPlatformVerification
     ..explicitExperimentalFlags = explicitExperimentalFlags
-    ..environmentDefines = noDefines ? null : parsedOptions.defines
+    ..environmentDefines = environmentDefines
     ..nnbdMode = nnbdMode
     ..enableUnscheduledExperiments = enableUnscheduledExperiments
     ..additionalDills = linkDependencies