Various issues with duplicated names.

pkg/front_end/lib/src/fasta/kernel/body_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.body_builder;
 
 import '../fasta_codes.dart'
     show FastaMessage, codeExpectedButGot, codeExpectedFunctionBody;
 
 import '../parser/parser.dart'
@@ skipping 600 matching lines @@
                   "Expected named argument.", arguments[i].fileOffset))
             ..fileOffset = beginToken.charOffset;
         }
       }
     }
     if (firstNamedArgumentIndex < arguments.length) {
       List<Expression> positional = new List<Expression>.from(
           arguments.getRange(0, firstNamedArgumentIndex));
       List<NamedExpression> named = new List<NamedExpression>.from(
           arguments.getRange(firstNamedArgumentIndex, arguments.length));
+      if (named.length == 2) {
+        if (named[0].name == named[1].name) {
+          named = <NamedExpression>[
+            new NamedExpression(
+                named[1].name,
+                buildCompileTimeError(
+                    "Duplicated named argument '${named[1].name}'.",
+                    named[1].fileOffset))
+          ];
+        }
+      } else if (named.length > 2) {
+        Map<String, NamedExpression> seenNames = <String, NamedExpression>{};
+        bool hasProblem = false;
+        for (NamedExpression expression in named) {
+          if (seenNames.containsKey(expression.name)) {
+            hasProblem = true;
+            seenNames[expression.name].value = buildCompileTimeError(
+                "Duplicated named argument '${expression.name}'.",
+                expression.fileOffset);
+          } else {
+            seenNames[expression.name] = expression;
+          }
+        }
+        if (hasProblem) {
+          named = new List<NamedExpression>.from(seenNames.values);
+        }
+      }
       push(new KernelArguments(positional, named: named)
         ..fileOffset = beginToken.charOffset);
     } else {
       push(new KernelArguments(arguments)..fileOffset = beginToken.charOffset);
     }
   }
 
   @override
   void handleParenthesizedExpression(BeginToken token) {
     debugEvent("ParenthesizedExpression");
@@ skipping 1578 matching lines @@
   }
 
   @override
   void endFunctionName(Token beginToken, Token token) {
     debugEvent("FunctionName");
     Identifier name = pop();
     VariableDeclaration variable = new KernelVariableDeclaration(
         name.name, functionNestingLevel,
         isFinal: true, isLocalFunction: true)
       ..fileOffset = offsetForToken(name.token);
+    if (scope.local[variable.name] != null) {
+      addCompileTimeError(offsetForToken(name.token),
+          "'${variable.name}' already declared in this scope.");
+    }
     push(new KernelFunctionDeclaration(
         variable,
         // The function node is created later.
         null)
       ..fileOffset = beginToken.charOffset);
     declareVariable(variable);
     enterLocalScope();
   }
 
   void enterFunction() {
@@ skipping 1282 matching lines @@
     if (starToken == null) {
       return AsyncMarker.Async;
     } else {
       assert(identical(starToken.stringValue, "*"));
       return AsyncMarker.AsyncStar;
     }
   } else {
     return internalError("Unknown async modifier: $asyncToken");
   }
 }