If an instruction is known to already by a number, the integer check will try to make it an integer.

lib/compiler/implementation/ssa/nodes.dart

 // Copyright (c) 2012, 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.
 
 interface HVisitor<R> {
   R visitAdd(HAdd node);
   R visitBailoutTarget(HBailoutTarget node);
   R visitBitAnd(HBitAnd node);
   R visitBitNot(HBitNot node);
   R visitBitOr(HBitOr node);
@@ skipping 1101 matching lines @@
 class HIntegerCheck extends HCheck {
   bool alwaysFalse = false;
 
   HIntegerCheck(value) : super(<HInstruction>[value]);
 
   HInstruction get value() => inputs[0];
   bool isControlFlow() => true;
 
   HType get guaranteedType() => HType.INTEGER;
 
+  HType computeDesiredTypeForInput(HInstruction input, HTypeMap types) {
+    // If the desired type of the input is already a number, we want
+    // to specialize it to an integer.
+    return input.isNumber(types)
+      ? HType.INTEGER
+      : super.computeDesiredTypeForInput(input, types);
+  }
+
   accept(HVisitor visitor) => visitor.visitIntegerCheck(this);
   int typeCode() => 3;
   bool typeEquals(other) => other is HIntegerCheck;
   bool dataEquals(HInstruction other) => true;
 }
 
 class HConditionalBranch extends HControlFlow {
   HConditionalBranch(inputs) : super(inputs);
   HInstruction get condition() => inputs[0];
   HBasicBlock get trueBranch() => block.successors[0];
@@ skipping 1611 matching lines @@
   HBasicBlock get start() => expression.start;
   HBasicBlock get end() {
     // We don't create a switch block if there are no cases.
     assert(!statements.isEmpty());
     return statements.last().end;
   }
 
   bool accept(HStatementInformationVisitor visitor) =>
       visitor.visitSwitchInfo(this);
 }