diff --git a/pkg/sourcemap_testing/lib/src/stacktrace_helper.dart b/pkg/sourcemap_testing/lib/src/stacktrace_helper.dart
index 7799b9ed6e1..8c545e2b480 100644
--- a/pkg/sourcemap_testing/lib/src/stacktrace_helper.dart
+++ b/pkg/sourcemap_testing/lib/src/stacktrace_helper.dart
@@ -436,12 +436,39 @@ class LineException {
 TargetEntry findEnclosingFunction(
     String sources, SourceFile file, int start, SingleMapping mapping) {
   if (sources == null) return null;
-  int index = sources.lastIndexOf(': function(', start);
-  if (index < 0) return null;
-  index += 2;
-  var line = file.getLine(index);
-  var lineEntry = _findLineInternal(mapping, line);
-  return _findColumn(line, file.getColumn(index), lineEntry);
+  var index = start;
+  while (true) {
+    index = nextDeclarationCandidate(sources, index);
+    if (index < 0) return null;
+    var line = file.getLine(index);
+    var lineEntry = _findLineInternal(mapping, line);
+    var result = _findColumn(line, file.getColumn(index), lineEntry);
+    if (result?.column == file.getColumn(index)) return result;
+    index--;
+  }
+}
+
+/// Returns the index of a candidate location of a enclosing function
+/// declaration. We try to find the beginning of a `function` keyword or the
+/// `(` of an ES6 method definition, but the search contains some false
+/// positives. To rule out false positives, [findEnclosingFunction]
+/// validates that the returned location contains a source-map entry, searching
+/// for another candidate if not.
+int nextDeclarationCandidate(String sources, int start) {
+  var indexForFunctionKeyword = sources.lastIndexOf('function', start);
+  // We attempt to identify potential method definitions by looking for any '('
+  // that precedes a '{'. This method will fail if 1) Dart2JS starts emitting
+  // functions with initializers or 2) sourcemap boundaries appear at '(' for
+  // non-method-definition constructs.
+  var indexForMethodDefinition = sources.lastIndexOf('{', start);
+  if (indexForFunctionKeyword > indexForMethodDefinition ||
+      indexForFunctionKeyword < 0) {
+    return indexForFunctionKeyword;
+  }
+  indexForMethodDefinition = sources.lastIndexOf('(', indexForMethodDefinition);
+  return indexForFunctionKeyword > indexForMethodDefinition
+      ? indexForFunctionKeyword
+      : indexForMethodDefinition;
 }
 
 Map<int, List<FrameEntry>> _loadInlinedFrameData(