[wasm_builder] Add method to deserialize just the sourceMapUrl

It is useful to be able to extract the sourceMapUrl without parsing
the rest of the module.

Change-Id: I595b615b07015597fc1b082aa320887d86f16204
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/455542
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Stephen Adams <[email protected]>

Author: rakudrama

pkg/wasm_builder/lib/src/ir/module.dart

@@ -119,7 +119,8 @@ class Module implements Serializable {
     SourceMapSection(sourceMapUrl).serialize(s);
   }
 
-  static Module deserialize(Deserializer d) {
+  static (Map<int, List<Deserializer>>, Map<String, List<Deserializer>>)
+      _deserializeTopLevel(Deserializer d) {
     final preamble = d.readBytes(8);
     if (preamble[0] != 0x00 ||
         preamble[1] != 0x61 ||
@@ -151,6 +152,12 @@ class Module implements Serializable {
       }
     }
 
+    return (sections, customSections);
+  }
+
+  static Module deserialize(Deserializer d) {
+    final (sections, customSections) = _deserializeTopLevel(d);
+
     final Module module = Module.uninitialized();
 
     // We read the sections in the order they should be in the binary.
@@ -235,6 +242,14 @@ class Module implements Serializable {
       );
   }
 
+  /// Deserialize just the `sourceMapUrl` section of a module as a [Uri].
+  static Uri? deserializeSourceMapUrl(Deserializer d) {
+    final (sections, customSections) = _deserializeTopLevel(d);
+    final sourceMapUrl = SourceMapSection.deserialize(
+        customSections[SourceMapSection.customSectionName]?.single);
+    return sourceMapUrl;
+  }
+
   String printAsWat() {
     final mp = ModulePrinter(this);