Support passing an environment variable map to child processes.

runtime/bin/process.dart

Index: runtime/bin/process.dart
diff --git a/runtime/bin/process.dart b/runtime/bin/process.dart
index dfe9ae9da0de90196b52b7825f0581ae245ce1e9..85699f264a91b1d8bc8f905aa6ec883020681f1d 100644
--- a/runtime/bin/process.dart
+++ b/runtime/bin/process.dart
@@ -141,6 +141,12 @@ class ProcessOptions {
    * The default stderrEncoding is UTF_8.
    */
   Encoding stderrEncoding;
+
+  /**
+   * Provides the environment variables for the process. If not set
+   * the environment of the parent process is inherited.
+   */
+  Map<String, String> environment;
 }