Don't crash 'pub get' if a dev dependency is broken.

sdk/lib/_internal/pub/lib/src/pubspec.dart

Index: sdk/lib/_internal/pub/lib/src/pubspec.dart
diff --git a/sdk/lib/_internal/pub/lib/src/pubspec.dart b/sdk/lib/_internal/pub/lib/src/pubspec.dart
index 6781a5c1df07afb8a298c3c1ea921cda03fabeff..c772402d8641f114c0fa2736ddb86cf8ae064fb2 100644
--- a/sdk/lib/_internal/pub/lib/src/pubspec.dart
+++ b/sdk/lib/_internal/pub/lib/src/pubspec.dart
@@ -96,9 +96,7 @@ class Pubspec {
   List<PackageDep> get dependencies {
     if (_dependencies != null) return _dependencies;
     _dependencies = _parseDependencies('dependencies');
-    if (_devDependencies == null) {
-      _checkDependencyOverlap(_dependencies, devDependencies);
-    }
+    _checkDependencyOverlap(_dependencies, _devDependencies);
     return _dependencies;
   }
   List<PackageDep> _dependencies;
@@ -107,9 +105,7 @@ class Pubspec {
   List<PackageDep> get devDependencies {
     if (_devDependencies != null) return _devDependencies;
     _devDependencies = _parseDependencies('dev_dependencies');
-    if (_dependencies == null) {
-      _checkDependencyOverlap(dependencies, _devDependencies);
-    }
+    _checkDependencyOverlap(_dependencies, _devDependencies);
     return _devDependencies;
   }
   List<PackageDep> _devDependencies;
@@ -334,7 +330,7 @@ class Pubspec {
           Iterable<PackageDep> dependencyOverrides,
           VersionConstraint sdkConstraint,
           Iterable<Iterable<TransformerConfig>> transformers,
-           Map fields, SourceRegistry sources})
+          Map fields, SourceRegistry sources})
       : _version = version,
         _dependencies = dependencies == null ? null : dependencies.toList(),
         _devDependencies = devDependencies == null ? null :
@@ -517,6 +513,9 @@ class Pubspec {
   /// dependency.
   void _checkDependencyOverlap(List<PackageDep> dependencies,
       List<PackageDep> devDependencies) {
+    if (dependencies == null) return;
+    if (devDependencies == null) return;
+
     var dependencyNames = dependencies.map((dep) => dep.name).toSet();
     var collisions = dependencyNames.intersection(
         devDependencies.map((dep) => dep.name).toSet());