[dart2wasm] Pass source map to wasm-opt when optimizing

To be able to know when we are generating a source map, make `dart
compile wasm` aware of the `--no-source-maps` flag.

Note: wasm-opt currently cannot handle the mappings we generate. This
will be merged after WebAssembly/binaryen#6794
and WebAssembly/binaryen#6795.

pkg/dartdev/lib/src/commands/compile.dart

@@ -707,6 +707,11 @@ class CompileWasmCommand extends CompileSubcommandCommand {
         },
         hide: !verbose,
       )
+      ..addFlag(
+        'no-source-maps',
+        help: 'Do not generate a source map file.',
+        negatable: false,
+      )
       ..addOption(
         packagesOption.flag,
         abbr: packagesOption.abbr,
@@ -814,6 +819,7 @@ class CompileWasmCommand extends CompileSubcommandCommand {
       if (args.flag('print-wasm')) '--print-wasm',
       if (args.flag('print-kernel')) '--print-kernel',
       if (args.flag('enable-asserts')) '--enable-asserts',
+      if (args.flag('no-source-maps')) '--no-source-maps',
       for (final define in defines) '-D$define',
       if (maxPages != null) ...[
         '--import-shared-memory',
@@ -843,14 +849,26 @@ class CompileWasmCommand extends CompileSubcommandCommand {
     }
 
     final bool strip = args.flag('strip-wasm');
+    final bool generateSourceMap = !args.flag('no-source-maps');
 
     if (runWasmOpt) {
       final unoptFile = '$outputFileBasename.unopt.wasm';
       File(outputFile).renameSync(unoptFile);
 
+      final unoptSourceMapFile = '$outputFileBasename.unopt.wasm.map';
+      if (generateSourceMap) {
+        File('$outputFile.map').renameSync(unoptSourceMapFile);
+      }
+
       final flags = [
         ...binaryenFlags,
         if (!strip) '-g',
+        if (generateSourceMap) ...[
+          '-ism',
+          unoptSourceMapFile,
+          '-osm',
+          '$outputFile.map'
+        ]
       ];
 
       if (verbose) {