Make pub strong-mode clean.

lib/src/lock_file.dart

Index: lib/src/lock_file.dart
diff --git a/lib/src/lock_file.dart b/lib/src/lock_file.dart
index b00bfdb6f9947f409af2473d0b88a7a2ff5a6d5b..3a19c2fccd6e69200db9072337894b9e5d8f74f2 100644
--- a/lib/src/lock_file.dart
+++ b/lib/src/lock_file.dart
@@ -76,32 +76,33 @@ class LockFile {
     var parsed = loadYamlNode(contents, sourceUrl: sourceUrl);
 
     _validate(parsed is Map, 'The lockfile must be a YAML mapping.', parsed);
+    var parsedMap = parsed as YamlMap;
 
     var dartSdkConstraint = VersionConstraint.any;
     VersionConstraint flutterSdkConstraint;
-    var sdkNode = parsed.nodes['sdk'];
+    var sdkNode = parsedMap.nodes['sdk'];
     if (sdkNode != null) {
       // Lockfiles produced by pub versions from 1.14.0 through 1.18.0 included
       // a top-level "sdk" field which encoded the unified constraint on the
       // Dart SDK. They had no way of specifying constraints on other SDKs.
       dartSdkConstraint = _parseVersionConstraint(sdkNode);
-    } else if ((parsed as Map).containsKey('sdks')) {
-      var sdksField = parsed['sdks'];
+    } else if (parsedMap.containsKey('sdks')) {
+      var sdksField = parsedMap['sdks'];
       _validate(
           sdksField is Map,
           'The "sdks" field must be a mapping.',
-          parsed.nodes['sdks']);
+          parsedMap.nodes['sdks']);
 
       dartSdkConstraint = _parseVersionConstraint(sdksField.nodes['dart']);
       flutterSdkConstraint =
           _parseVersionConstraint(sdksField.nodes['flutter']);
     }
 
-    var packages = {};
-    var packageEntries = parsed['packages'];
+    var packages = <String, PackageId>{};
+    var packageEntries = parsedMap['packages'];
     if (packageEntries != null) {
       _validate(packageEntries is Map, 'The "packages" field must be a map.',
-          parsed.nodes['packages']);
+          parsedMap.nodes['packages']);
 
       packageEntries.forEach((name, spec) {
         // Parse the version.
@@ -182,7 +183,7 @@ class LockFile {
   LockFile setPackage(PackageId id) {
     if (id.isRoot) return this;
 
-    var packages = new Map.from(this.packages);
+    var packages = new Map<String, PackageId>.from(this.packages);
     packages[id.name] = id;
     return new LockFile._(packages, dartSdkConstraint, flutterSdkConstraint);
   }
@@ -193,7 +194,7 @@ class LockFile {
   LockFile removePackage(String name) {
     if (!this.packages.containsKey(name)) return this;
 
-    var packages = new Map.from(this.packages);
+    var packages = new Map<String, PackageId>.from(this.packages);
     packages.remove(name);
     return new LockFile._(packages, dartSdkConstraint, flutterSdkConstraint);
   }
@@ -205,7 +206,8 @@ class LockFile {
   String packagesFile(SystemCache cache, [String entrypoint]) {
     var header = "Generated by pub on ${new DateTime.now()}.";
 
-    var map = new Map.fromIterable(ordered(packages.keys), value: (name) {
+    var map = new Map<String, Uri>.fromIterable(ordered(packages.keys),
+        value: (name) {
       var id = packages[name];
       var source = cache.source(id.source);
       return p.toUri(p.join(source.getDirectory(id), "lib"));