Canonicalize mixin applications by their name

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.kernel_mixin_application_builder;
 
 import 'package:kernel/ast.dart' show
     InterfaceType,
     Supertype,
     setParents;
@@ skipping 46 matching lines @@
     KernelTypeBuilder s = this.supertype;
     for (KernelTypeBuilder builder in mixins) {
       s = applyMixin(s, builder);
     }
     builtType = s.buildSupertype();
     return builtType;
   }
 
   TypeBuilder applyMixin(TypeBuilder supertype, TypeBuilder mixin) {
     KernelLibraryBuilder library = this.library.partOfLibrary ?? this.library;
+    List<KernelTypeBuilder> typeArguments;
     List<TypeVariableBuilder> newTypeVariables;
-    List<KernelTypeBuilder> typeArguments;
     if (typeVariables != null) {
+      assert(subclassName != null);
       newTypeVariables = library.copyTypeVariables(typeVariables);
       Map<TypeVariableBuilder, TypeBuilder> substitution =
           <TypeVariableBuilder, TypeBuilder>{};
       typeArguments = <KernelTypeBuilder>[];
       for (int i = 0; i < typeVariables.length; i++) {
         substitution[typeVariables[i]] = newTypeVariables[i].asTypeBuilder();
         typeArguments.add(typeVariables[i].asTypeBuilder());
       }
       supertype = supertype.subst(substitution);
       mixin = mixin.subst(substitution);
     }
     // To reduce diff against dartk, we create a different name for mixin
     // applications that have free type variables. We do this by setting
     // [subclassName] when setting typeVariables.
     String name = subclassName != null
         ? "${subclassName}^${mixin.name}"
         : "${supertype.name}&${mixin.name}";
-    SourceClassBuilder cls = new SourceClassBuilder(null, abstractMask, name,
-        newTypeVariables, supertype, null, <String, Builder>{}, library,
-        <ConstructorReferenceBuilder>[], charOffset, null, mixin);
-    library.addImplementationBuilder(name, cls, charOffset);
-    if (newTypeVariables != null) {
-      for (KernelTypeVariableBuilder t in newTypeVariables) {
-        cls.cls.typeParameters.add(t.parameter);
+
+    SourceClassBuilder cls =
+        library.mixinApplicationClasses.putIfAbsent(name, () {
+      SourceClassBuilder cls = new SourceClassBuilder(
+          null,
+          abstractMask,
+          name,
+          newTypeVariables,
+          supertype,
+          null,
+          <String, Builder>{},
+          library,
+          <ConstructorReferenceBuilder>[],
+          charOffset,
+          null,
+          mixin);
+      library.addImplementationBuilder(name, cls, charOffset);
+      if (newTypeVariables != null) {
+        for (KernelTypeVariableBuilder t in newTypeVariables) {
+          cls.cls.typeParameters.add(t.parameter);
+        }
+        setParents(cls.cls.typeParameters, cls.cls);
       }
-      setParents(cls.cls.typeParameters, cls.cls);
-    }
-    return new KernelNamedTypeBuilder(name, typeArguments, charOffset,
-        library.fileUri)
-        ..builder = cls;
+      return cls;
+    });
+    return new KernelNamedTypeBuilder(
+        name, typeArguments, charOffset, library.fileUri)..builder = cls;
   }
 }