Document dart2js flags in dartjs -h

pkg/compiler/lib/src/dart2js.dart

Index: pkg/compiler/lib/src/dart2js.dart
diff --git a/pkg/compiler/lib/src/dart2js.dart b/pkg/compiler/lib/src/dart2js.dart
index 55d21e2e9acddedc8fd94310ba083c489f2144f9..8f6742ab3b25242d0dfe0d53245d801809034b86 100644
--- a/pkg/compiler/lib/src/dart2js.dart
+++ b/pkg/compiler/lib/src/dart2js.dart
@@ -744,6 +744,31 @@ Supported options:
   --no-source-maps
     Do not generate a source map file.
 
+  --fast-startup
+    Produce JavaScript that can be parsed more quickly by VMs. This option
+    usually results in larger JavaScript files with faster startup.
+    Note: the dart:mirrors library is not supported with this option.
+
+The following advanced options can help reduce the size of the generated code,
+but they may cause programs to behave unexpectedly if assumptions are not met.
+Only turn on these flags if you have enough test coverage to ensure they are
+safe to use:
+
+  --trust-type-annotations
+    Assume that all types are correct. This option allows the compiler to drop
+    type checks and to rely on local type information for optimizations. Use
+    this option only if you have enough testing to ensure that your program
+    works in strong mode or checked mode.
+
+  --trust-primitives
+    Assume that operations on numbers, strings, and lists have valid inputs.
+    This option allows the compiler to drop runtime checks for those operations.
+    Note: a well-typed program is not guaranteed to have valid inputs. For
+    example, an int index argument may be null or out of range.
+
+    Use this option only if you have enough testing to ensure that those checks

[Kathy Walrath, 2017/07/10 19:55:06]

Delete this paragraph? If not, then delete the second sentence of the intro
paragraph above ("Only turn on these flags if...").

[Siggi Cherem (dart-lang), 2017/07/10 19:57:54]

Done.

+    are not needed; otherwise the program will behave unexpectedly.
+
 The following options are only used for compiler development and may
 be removed in a future version: