Add support for compiling Dart via the FE in dart2js.

pkg/compiler/lib/src/options.dart

Index: pkg/compiler/lib/src/options.dart
diff --git a/pkg/compiler/lib/src/options.dart b/pkg/compiler/lib/src/options.dart
index 169fce92231597d20be679354286499a20f3abe3..5b0088d33ffc97ce8c73cfa01c278c5f4cbc0eb3 100644
--- a/pkg/compiler/lib/src/options.dart
+++ b/pkg/compiler/lib/src/options.dart
@@ -202,9 +202,17 @@ class CompilerOptions implements DiagnosticOptions {
   /// Whether to use kernel internally as part of compilation.
   final bool useKernel;
 
-  /// Read input from a .dill file rather than a .dart input (only to be used in
-  /// conjunction with useKernel = true).
-  final bool loadFromDill;
+  /// Preview the unified front-end and compilation from kernel.
+  ///
+  /// When enabled the compiler will use the unified front-end to compile
+  /// sources to kernel, and then continue compilation from the kernel
+  /// representation. Setting this flag will implicitly set [useKernel] to true
+  /// as well.
+  ///
+  /// When this flag is on, the compiler also acccepts reading .dill files from
+  /// disk. The compiler reads the sources differently depending on the
+  /// extension format.
+  final bool previewDart2;
 
   // Whether to use kernel internally for global type inference calculations.
   // TODO(efortuna): Remove this and consolidate with useKernel.
@@ -324,7 +332,7 @@ class CompilerOptions implements DiagnosticOptions {
         useContentSecurityPolicy:
             _hasOption(options, Flags.useContentSecurityPolicy),
         useKernel: _hasOption(options, Flags.useKernel),
-        loadFromDill: _hasOption(options, Flags.loadFromDill),
+        previewDart2: _hasOption(options, Flags.previewDart2),
         useFrequencyNamer:
             !_hasOption(options, Flags.noFrequencyBasedMinification),
         useMultiSourceInfo: _hasOption(options, Flags.useMultiSourceInfo),
@@ -387,7 +395,7 @@ class CompilerOptions implements DiagnosticOptions {
       bool trustTypeAnnotations: false,
       bool useContentSecurityPolicy: false,
       bool useKernel: false,
-      bool loadFromDill: false,
+      bool previewDart2: false,
       bool useFrequencyNamer: true,
       bool useMultiSourceInfo: false,
       bool useNewSourceInfo: false,
@@ -432,8 +440,10 @@ class CompilerOptions implements DiagnosticOptions {
         suppressWarnings: suppressWarnings,
         suppressHints: suppressHints,
         shownPackageWarnings: shownPackageWarnings,
-        disableInlining: disableInlining,
-        disableTypeInference: disableTypeInference,
+        // TODO(sigmund): remove once we support inlining and type-inference
+        // with `previewDart2`.
+        disableInlining: disableInlining || previewDart2,
+        disableTypeInference: disableTypeInference || previewDart2,
         dumpInfo: dumpInfo,
         enableAssertMessage: enableAssertMessage,
         enableExperimentalMirrors: enableExperimentalMirrors,
@@ -443,7 +453,8 @@ class CompilerOptions implements DiagnosticOptions {
         enableUserAssertions: enableUserAssertions,
         experimentalTrackAllocations: experimentalTrackAllocations,
         experimentalAllocationsPath: experimentalAllocationsPath,
-        generateCodeWithCompileTimeErrors: generateCodeWithCompileTimeErrors,
+        generateCodeWithCompileTimeErrors:
+            generateCodeWithCompileTimeErrors && !previewDart2,
         generateSourceMap: generateSourceMap,
         kernelGlobalInference: kernelGlobalInference,
         outputUri: outputUri,
@@ -461,8 +472,8 @@ class CompilerOptions implements DiagnosticOptions {
         trustPrimitives: trustPrimitives,
         trustTypeAnnotations: trustTypeAnnotations,
         useContentSecurityPolicy: useContentSecurityPolicy,
-        useKernel: useKernel,
-        loadFromDill: loadFromDill,
+        useKernel: useKernel || previewDart2,

[Emily Fortuna, 2017/07/21 21:50:51]

can we delete this option, too?

[Siggi Cherem (dart-lang), 2017/07/22 00:04:04]

Great question - we chatted a bit about this offline and I was quite ambivalent
at first. I lean towards keeping the two flags for a little longer because we
are not getting much coverage from the ssa builder otherwise (in particular, we
need to fix an issue with the `FE` not allowing dart:_internal imports in tests
(all dart2js_native tests fail for this reason), and land a few other changes in
the pipeline). Let's chat more about this when we are all together next week
though.

+        previewDart2: previewDart2,
         useFrequencyNamer: useFrequencyNamer,
         useMultiSourceInfo: useMultiSourceInfo,
         useNewSourceInfo: useNewSourceInfo,
@@ -515,7 +526,7 @@ class CompilerOptions implements DiagnosticOptions {
       this.trustTypeAnnotations: false,
       this.useContentSecurityPolicy: false,
       this.useKernel: false,
-      this.loadFromDill: false,
+      this.previewDart2: false,
       this.useFrequencyNamer: false,
       this.useMultiSourceInfo: false,
       this.useNewSourceInfo: false,
@@ -575,7 +586,7 @@ class CompilerOptions implements DiagnosticOptions {
       trustTypeAnnotations,
       useContentSecurityPolicy,
       useKernel,
-      loadFromDill,
+      previewDart2,
       useFrequencyNamer,
       useMultiSourceInfo,
       useNewSourceInfo,
@@ -647,7 +658,7 @@ class CompilerOptions implements DiagnosticOptions {
         useContentSecurityPolicy:
             useContentSecurityPolicy ?? options.useContentSecurityPolicy,
         useKernel: useKernel ?? options.useKernel,
-        loadFromDill: loadFromDill ?? options.loadFromDill,
+        previewDart2: previewDart2 ?? options.previewDart2,
         useFrequencyNamer: useFrequencyNamer ?? options.useFrequencyNamer,
         useMultiSourceInfo: useMultiSourceInfo ?? options.useMultiSourceInfo,
         useNewSourceInfo: useNewSourceInfo ?? options.useNewSourceInfo,