compatibleWith(): implement toString() and use to parse ^

pkgs/pub_semver/lib/src/version_constraint.dart

diff --git a/pkgs/pub_semver/lib/src/version_constraint.dart b/pkgs/pub_semver/lib/src/version_constraint.dart
index 0cd45b6..f67375f 100644
--- a/pkgs/pub_semver/lib/src/version_constraint.dart
+++ b/pkgs/pub_semver/lib/src/version_constraint.dart
@@ -112,8 +112,7 @@
             '"^" constraint "$constraint" in "$originalText".');
       }
 
-      return new VersionRange(min: version, includeMin: true,
-          max: version.nextBreaking);
+      return new VersionConstraint.compatibleWith(version);
     }
 
     while (true) {
@@ -156,10 +155,8 @@
   /// Versions are considered backward compatible with [version] if they
   /// are greater than or equal to [version], but less than the next breaking
   /// version ([Version.nextBreaking]) of [version].
-  factory VersionConstraint.compatibleWith(Version version) {
-    return new VersionRange(min: version, includeMin: true,
-        max: version.nextBreaking);
-  }
+  factory VersionConstraint.compatibleWith(Version version) =>
+      new _CompatibleWithVersionRange(version);
 
   /// Creates a new version constraint that is the intersection of
   /// [constraints].
@@ -199,3 +196,11 @@
   VersionConstraint intersect(VersionConstraint other) => this;
   String toString() => '<empty>';
 }
+
+class _CompatibleWithVersionRange extends VersionRange {
+  _CompatibleWithVersionRange(Version version) : super(
+      min: version, includeMin: true,
+      max: version.nextBreaking, includeMax: false);
+
+  String toString() => '^$min';
+}

pkgs/pub_semver/test/version_constraint_test.dart

diff --git a/pkgs/pub_semver/test/version_constraint_test.dart b/pkgs/pub_semver/test/version_constraint_test.dart
index 316d57d..9ba6905 100644
--- a/pkgs/pub_semver/test/version_constraint_test.dart
+++ b/pkgs/pub_semver/test/version_constraint_test.dart
@@ -174,10 +174,18 @@
     });
   });
 
-  test('compatibleWith', () {
-    var constraint = new VersionConstraint.compatibleWith(v072);
+  group('compatibleWith()', () {
+    test('returns the range of compatible versions', () {
+      var constraint = new VersionConstraint.compatibleWith(v072);
 
-    expect(constraint, equals(new VersionRange(min: v072, includeMin: true,
-        max: v072.nextBreaking)));
+      expect(constraint, equals(new VersionRange(min: v072, includeMin: true,
+          max: v072.nextBreaking)));
+    });
+
+    test('toString() uses "^"', () {
+      var constraint = new VersionConstraint.compatibleWith(v072);
+
+      expect(constraint.toString(), equals('^0.7.2'));
+    });
   });
 }