double * int returns a double can be simplified to an int if both operands are constants. Therefore…

sdk/lib/_internal/compiler/implementation/ssa/optimize.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.
 
 part of ssa;
 
 abstract class OptimizationPhase {
   String get name;
   void visitGraph(HGraph graph);
 }
@@ skipping 105 matching lines @@
   }
 
   visitBasicBlock(HBasicBlock block) {
     HInstruction instruction = block.first;
     while (instruction != null) {
       HInstruction next = instruction.next;
       HInstruction replacement = instruction.accept(this);
       if (replacement != instruction) {
         block.rewrite(instruction, replacement);
 
-        // If we can replace [instruction] with [replacement], then
-        // [replacement]'s type can be narrowed.
-        TypeMask newType = replacement.instructionType.intersection(
-            instruction.instructionType, compiler);
-        replacement.instructionType = newType;
+        // The intersection of double and int return conflicting, and
+        // because of our number implementation for JavaScript, it
+        // might be that an operation thought to return double, can be
+        // simplified to an int. For example:
+        // `2.5 * 10`.
+        if (!(replacement.isNumberOrNull(compiler)
+              && instruction.isNumberOrNull(compiler))) {
+          // If we can replace [instruction] with [replacement], then
+          // [replacement]'s type can be narrowed.
+          TypeMask newType = replacement.instructionType.intersection(
+              instruction.instructionType, compiler);
+          replacement.instructionType = newType;
+        }
 
         // If the replacement instruction does not know its
         // source element, use the source element of the
         // instruction.
         if (replacement.sourceElement == null) {
           replacement.sourceElement = instruction.sourceElement;
         }
         if (replacement.sourcePosition == null) {
           replacement.sourcePosition = instruction.sourcePosition;
         }
@@ skipping 1800 matching lines @@
 
     keyedValues.forEach((receiver, values) {
       result.keyedValues[receiver] =
           new Map<HInstruction, HInstruction>.from(values);
     });
 
     result.nonEscapingReceivers.addAll(nonEscapingReceivers);
     return result;
   }
 }