[kernel] Partial support for conditional imports in CFE.

In order to un-block Flutter integration, we introduce partial support for
conditional imports into the CFE. There are two incomplete parts:

- The condition strings are evaluated by the Target.
  They should be looked up in the environment, but this introduces
  complications with modular compilation.

- Type inference and other static checks are performed with reference to
  the implementation (conditionally-imported) library rather than the
  interface library.

See issue #30143 for more details.

Bug:
Change-Id: I740b45e9d32796644837de4caefd8d6e8015f229
Reviewed-on: https://dart-review.googlesource.com/34721
Reviewed-by: Peter von der Ahé <ahe@google.com>

pkg/front_end/lib/src/fasta/configuration.dart

@@ -0,0 +1,14 @@
+// Copyright (c) 2018, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library fasta.configuration;
+
+class Configuration {
+  final int charOffset;
+  final String dottedName;
+  final String condition;
+  final String importUri;
+  Configuration(
+      this.charOffset, this.dottedName, this.condition, this.importUri);
+}

pkg/front_end/lib/src/fasta/import.dart

@@ -12,6 +12,8 @@ import 'kernel/kernel_builder.dart' show toKernelCombinators;
 
 import 'combinator.dart' show Combinator;
 
+import 'configuration.dart' show Configuration;
+
 typedef void AddToScope(String name, Builder member);
 
 class Import {
@@ -29,12 +31,21 @@ class Import {
 
   final List<Combinator> combinators;
 
+  final List<Configuration> configurations;
+
   final int charOffset;
 
   final int prefixCharOffset;
 
-  Import(this.importer, this.imported, this.deferred, this.prefix,
-      this.combinators, this.charOffset, this.prefixCharOffset)
+  Import(
+      this.importer,
+      this.imported,
+      this.deferred,
+      this.prefix,
+      this.combinators,
+      this.configurations,
+      this.charOffset,
+      this.prefixCharOffset)
       : prefixBuilder = createPrefixBuilder(prefix, importer, imported,
             combinators, deferred, charOffset, prefixCharOffset);
 

pkg/front_end/lib/src/fasta/source/outline_builder.dart

@@ -44,7 +44,9 @@ import '../quote.dart' show unescapeString;
 
 import 'source_library_builder.dart' show SourceLibraryBuilder;
 
-import 'unhandled_listener.dart' show NullValue, Unhandled, UnhandledListener;
+import 'unhandled_listener.dart' show NullValue, UnhandledListener;
+
+import '../configuration.dart' show Configuration;
 
 enum MethodBody {
   Abstract,
@@ -143,7 +145,7 @@ class OutlineBuilder extends UnhandledListener {
   void endExport(Token exportKeyword, Token semicolon) {
     debugEvent("Export");
     List<Combinator> combinators = pop();
-    Unhandled conditionalUris = pop();
+    List<Configuration> conditionalUris = pop();
     int uriOffset = popCharOffset();
     String uri = pop();
     List<MetadataBuilder> metadata = pop();
@@ -171,15 +173,38 @@ class OutlineBuilder extends UnhandledListener {
     bool isDeferred = pop();
     int prefixOffset = pop();
     String prefix = pop(NullValue.Prefix);
-    Unhandled conditionalUris = pop();
+    List<Configuration> configurations = pop();
     int uriOffset = popCharOffset();
-    String uri = pop();
+    String uri = pop(); // For a conditional import, this is the default URI.
     List<MetadataBuilder> metadata = pop();
-    library.addImport(metadata, uri, conditionalUris, prefix, combinators,
+    library.addImport(metadata, uri, configurations, prefix, combinators,
         isDeferred, importKeyword.charOffset, prefixOffset, uriOffset);
     checkEmpty(importKeyword.charOffset);
   }
 
+  @override
+  void endConditionalUris(int count) {
+    debugEvent("EndConditionalUris");
+    push(popList(count) ?? NullValue.ConditionalUris);
+  }
+
+  @override
+  void endConditionalUri(Token ifKeyword, Token leftParen, Token equalSign) {
+    debugEvent("EndConditionalUri");
+    int charOffset = popCharOffset();
+    String uri = pop();
+    if (equalSign != null) popCharOffset();
+    String condition = popIfNotNull(equalSign) ?? "true";
+    String dottedName = pop();
+    push(new Configuration(charOffset, dottedName, condition, uri));
+  }
+
+  @override
+  void handleDottedName(int count, Token firstIdentifier) {
+    debugEvent("DottedName");
+    push(popIdentifierList(count).join('.'));
+  }
+
   @override
   void handleRecoverImport(Token semicolon) {
     debugEvent("RecoverImport");

pkg/front_end/lib/src/fasta/source/source_library_builder.dart

@@ -30,7 +30,6 @@ import '../builder/builder.dart'
         TypeBuilder,
         TypeDeclarationBuilder,
         TypeVariableBuilder,
-        Unhandled,
         UnresolvedType;
 
 import '../combinator.dart' show Combinator;
@@ -58,6 +57,8 @@ import '../fasta_codes.dart'
 
 import '../import.dart' show Import;
 
+import '../configuration.dart' show Configuration;
+
 import '../problems.dart' show unhandled;
 
 import 'source_loader.dart' show SourceLoader;
@@ -191,7 +192,7 @@ abstract class SourceLibraryBuilder<T extends TypeBuilder, R>
   void addExport(
       List<MetadataBuilder> metadata,
       String uri,
-      Unhandled conditionalUris,
+      List<Configuration> conditionalUris,
       List<Combinator> combinators,
       int charOffset,
       int uriOffset) {
@@ -204,13 +205,24 @@ abstract class SourceLibraryBuilder<T extends TypeBuilder, R>
   void addImport(
       List<MetadataBuilder> metadata,
       String uri,
-      Unhandled conditionalUris,
+      List<Configuration> configurations,
       String prefix,
       List<Combinator> combinators,
       bool deferred,
       int charOffset,
       int prefixCharOffset,
       int uriOffset) {
+    if (configurations != null) {
+      for (Configuration config in configurations) {
+        if (loader.target.backendTarget
+                .lookupImportCondition(config.dottedName) ==
+            config.condition) {
+          uri = config.importUri;
+          break;
+        }
+      }
+    }
+
     imports.add(new Import(
         this,
         loader.read(resolve(this.uri, uri, uriOffset), charOffset,
@@ -218,6 +230,7 @@ abstract class SourceLibraryBuilder<T extends TypeBuilder, R>
         deferred,
         prefix,
         combinators,
+        configurations,
         charOffset,
         prefixCharOffset));
   }

pkg/front_end/lib/src/fasta/source/stack_listener.dart

@@ -34,6 +34,7 @@ enum NullValue {
   Combinators,
   Comments,
   ConditionalUris,
+  ConditionallySelectedImport,
   ConstructorInitializerSeparator,
   ConstructorInitializers,
   ConstructorReferenceContinuationAfterTypeArguments,

pkg/kernel/lib/target/targets.dart

@@ -152,6 +152,10 @@ abstract class Target {
   /// extensions, but targets like dart2js only enable it on the core libraries.
   bool enableNative(Uri uri) => false;
 
+  /// Returns whether the given condition Uri should evaluate to true for the
+  /// purposes of resolving conditional imports.
+  String lookupImportCondition(String condition) => "";
+
   /// There are two variants of the `native` language extension. The VM expects
   /// the native token to be followed by string, whereas dart2js and DDC do not.
   // TODO(sigmund, ahe): ideally we should remove the `native` syntax, if not,