Add a validator for dependency version constraints.

utils/pub/entrypoint.dart

Index: utils/pub/entrypoint.dart
diff --git a/utils/pub/entrypoint.dart b/utils/pub/entrypoint.dart
index 56007e0fe78dd90ab32c0a88af95cdaae972a4be..3f3a57fd08709345d2245f210dfb14825a8c8d2b 100644
--- a/utils/pub/entrypoint.dart
+++ b/utils/pub/entrypoint.dart
@@ -114,7 +114,7 @@ class Entrypoint {
    * completes when all dependencies are installed.
    */
   Future installDependencies() {
-    return _loadLockFile()
+    return loadLockFile()
       .chain((lockFile) => resolveVersions(cache.sources, root, lockFile))
       .chain(_installDependencies);
   }
@@ -135,7 +135,7 @@ class Entrypoint {
    * [Future] that completes when all dependencies are installed.
    */
   Future updateDependencies(List<String> dependencies) {
-    return _loadLockFile().chain((lockFile) {
+    return loadLockFile().chain((lockFile) {
       var versionSolver = new VersionSolver(cache.sources, root, lockFile);
       for (var dependency in dependencies) {
         versionSolver.useLatestVersion(dependency);
@@ -159,14 +159,9 @@ class Entrypoint {
       .chain(_linkSecondaryPackageDirs);
   }
 
-  /**
-   * Loads the list of concrete package versions from the `pubspec.lock`, if it
-   * exists. If it doesn't, this completes to an empty [LockFile].
-   *
-   * If there's an error reading the `pubspec.lock` file, this will print a
-   * warning message and act as though the file doesn't exist.
-   */
-  Future<LockFile> _loadLockFile() {
+  /// Loads the list of concrete package versions from the `pubspec.lock`, if it
+  /// exists. If it doesn't, this completes to an empty [LockFile].
+  Future<LockFile> loadLockFile() {
     var lockFilePath = join(root.dir, 'pubspec.lock');
 
     log.fine("Loading lockfile.");