Handle bad factory names correctly.

pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
index 4eac6a14505866ac4bb8f5f3d6b00eae7530421d..8c0031c4b81aa67282c039d66ff3cba7cf755dac 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
@@ -153,6 +153,12 @@ class KernelLibraryBuilder
         charOffset);
   }
 
+  String computeConstructorName(String name) {
+    assert(isConstructorName(name, currentDeclaration.name));
+    int index = name.indexOf(".");
+    return index == -1 ? "" : name.substring(index + 1);
+  }
+
   void addProcedure(
       List<MetadataBuilder> metadata,
       int modifiers,
@@ -171,8 +177,7 @@ class KernelLibraryBuilder
     endNestedDeclaration().resolveTypes(typeVariables, this);
     ProcedureBuilder procedure;
     if (!isTopLevel && isConstructorName(name, currentDeclaration.name)) {
-      int index = name.indexOf(".");
-      name = index == -1 ? "" : name.substring(index + 1);
+      name = computeConstructorName(name);
       procedure = new KernelConstructorBuilder(
           metadata,
           modifiers & ~abstractMask,
@@ -219,8 +224,9 @@ class KernelLibraryBuilder
     DeclarationBuilder<KernelTypeBuilder> factoryDeclaration =
         endNestedDeclaration();
     String name = constructorName.name;
-    int index = name.indexOf(".");
-    name = index == -1 ? "" : name.substring(index + 1);
+    if (isConstructorName(name, currentDeclaration.name)) {
+      name = computeConstructorName(name);
+    }
     assert(constructorName.suffix == null);
     KernelProcedureBuilder procedure = new KernelProcedureBuilder(
         metadata,