Implement compareTo on DartC.

compiler/lib/implementation/number.dart

 // Copyright (c) 2011, 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.
 
 class NumberImplementation implements int, double native "Number" {
   NumberImplementation operator +(NumberImplementation other) native;
   NumberImplementation operator -(NumberImplementation other) native;
   NumberImplementation operator *(NumberImplementation other) native;
   NumberImplementation operator /(NumberImplementation other) native;
   NumberImplementation operator ~/(NumberImplementation other) native;
@@ skipping 14 matching lines @@
 
   NumberImplementation remainder(num other) native;
   NumberImplementation abs() native;
   NumberImplementation round() native;
   NumberImplementation floor() native;
   NumberImplementation ceil() native;
   NumberImplementation truncate() native;
 
   // CompareTo has to give a complete order, including -0/+0, NaN and
   // Infinities.
+  // Order is: -Inf < .. < -0.0 < 0.0 .. < +inf < NaN.
   NumberImplementation compareTo(NumberImplementation other) {
-    // TODO(5427706): NumberImplementation.compareTo is broken, since it
-    // doesn't take NaNs and -0.0 into account.
-    // And it doesn't return an int...
-    return this - other;
+    // Don't use the 'this' object (which is a JS Number object), but get the
+    // primitive JS number by invoking toDouble().
+    num thisValue = toDouble();
+    // Remember that NaN return false for any comparison.
+    if (thisValue < other) {
+      return -1;
+    } else if (thisValue > other) {
+      return 1;
+    } else if (thisValue == other) {
+      if (thisValue == 0) {
+        bool thisIsNegative = isNegative();
+        bool otherIsNegative = other.isNegative();
+        if (thisIsNegative == otherIsNegative) return 0;
+        if (thisIsNegative) return -1;
+        return 1;
+      }
+      return 0;
+    } else if (isNaN()) {
+      if (other.isNaN()) {
+        return 0;
+      }
+      return 1;
+    } else {
+      return -1;
+    }
   }
 
   bool isNegative() native;
   bool isEven() native;
   bool isOdd() native;
   bool isNaN() native;
   bool isInfinite() native;
 
   int toInt() {
     if (isNaN()) throw new BadNumberFormatException("NaN");
     if (isInfinite()) throw new BadNumberFormatException("Infinity");
     NumberImplementation truncated = truncate();
     // If truncated is -0.0 return +0. The test will also trigger for positive
     // 0s but that's not a problem.
     if (truncated == -0.0) return 0;
     return truncated;
   }
 
   NumberImplementation toDouble() native;
   String toString() native;
   String toStringAsFixed(int fractionDigits) native;
   String toStringAsExponential(int fractionDigits) native;
   String toStringAsPrecision(int precision) native;
   String toRadixString(int radix) native;
 
   int hashCode() native;
   get dynamic() { return toDouble(); }
 }