Don't resynthesize field types during class members resynthesis.

pkg/analyzer/lib/src/summary/summarize_ast.dart

Index: pkg/analyzer/lib/src/summary/summarize_ast.dart
diff --git a/pkg/analyzer/lib/src/summary/summarize_ast.dart b/pkg/analyzer/lib/src/summary/summarize_ast.dart
index ce0307d1bb984e2ebffae41e4c5da86b7cb27445..240fa8000cae17a4614eb07cb90d362c4ba6ccb7 100644
--- a/pkg/analyzer/lib/src/summary/summarize_ast.dart
+++ b/pkg/analyzer/lib/src/summary/summarize_ast.dart
@@ -710,8 +710,10 @@ class _SummarizeAstVisitor extends RecursiveAstVisitor {
       bool serializeBody) {
     if (body is BlockFunctionBody || body is ExpressionFunctionBody) {
       for (UnlinkedParamBuilder parameter in b.parameters) {
-        parameter.visibleOffset = body.offset;
-        parameter.visibleLength = body.length;
+        if (!parameter.isInitializingFormal) {
+          parameter.visibleOffset = body.offset;
+          parameter.visibleLength = body.length;
+        }
       }
     }
     List<UnlinkedExecutableBuilder> oldExecutables = executables;