[et] Add support for building Dart SDK from source

Author: dcharkes

tools/engine_tool/lib/src/commands/build_command.dart

@@ -31,6 +31,7 @@ final class BuildCommand extends CommandBase {
     );
     addConcurrencyOption(argParser);
     addRbeOptions(argParser, environment);
+    addDartBuildOptions(argParser, environment);
     argParser.addFlag(
       ltoFlag,
       help: 'Whether LTO should be enabled for a build. Default is disabled',
@@ -75,9 +76,14 @@ et build //flutter/fml:fml_benchmarks  # Build a specific target in `//flutter/f
       return 1;
     }
 
+    final bool buildDartFull = argResults![buildDartFullFlag] as bool;
+    final bool buildDart = argResults![buildDartFlag] as bool || buildDartFull;
+
     final List<String> extraGnArgs = <String>[
       if (!useRbe) '--no-rbe',
       if (useLto) '--lto' else '--no-lto',
+      if (buildDart) '--no-prebuilt-dart-sdk',
+      if (buildDartFull) '--full-dart-sdk',
     ];
 
     final List<String> commandLineTargets = argResults!.rest;

tools/engine_tool/lib/src/commands/command.dart

@@ -94,3 +94,15 @@ void addRbeOptions(ArgParser parser, Environment environment) {
     defaultsTo: buildStrategyFlagValueAuto,
   );
 }
+
+/// Adds the --build-dart and --build-dart-full flags.
+void addDartBuildOptions(ArgParser parser, Environment environment) {
+  parser.addFlag(
+    buildDartFlag,
+    help: 'Build Dart from source',
+  );
+  parser.addFlag(
+    buildDartFullFlag,
+    help: 'Build the full Dart SDK from source',
+  );
+}

tools/engine_tool/lib/src/commands/flags.dart

@@ -12,6 +12,8 @@
 
 // Keep this list alphabetized.
 const String allFlag = 'all';
+const String buildDartFlag = 'build-dart';
+const String buildDartFullFlag = 'build-dart-full';
 const String builderFlag = 'builder';
 const String concurrencyFlag = 'concurrency';
 const String configFlag = 'config';