[vm] Add examples to heapsnapshot tool

Change-Id: I2b3735341146e532a747e302050e3983ee8f7e1c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/273240
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>

runtime/tools/heapsnapshot/README

@@ -0,0 +1,67 @@
+Examples:
+
+# Load
+load <file>
+
+# Show named sets / known variables
+info
+
+# Find live objects
+closure roots
+
+# or give a name
+all = closure roots
+
+# Look into the object
+stats all
+
+# Filter lists from "all" into "lists"
+lists = filter all _List
+
+# Find empty lists into "empty-lists"
+empty-lists = dfilter lists ==0
+
+# Who's using the empty lists?
+users empty-lists
+
+# print that info (from $0 in this case as we didn't give it a name but it was
+# the first one we didn't give a name)
+stat $0
+
+# Filter more
+empty-growable-lists = filter (users empty-lists) _GrowableList
+
+# Print
+stats empty-growable-lists
+
+# Who's using them?
+retainers empty-growable-lists
+
+# Look into strings next
+strings = filter all String
+
+# What's inside the strings
+dstats strings
+
+# Who's pointing to the big strings?
+retainers (dfilter strings >=1024)
+
+# Small strings
+small-strings = dfilter strings <100
+
+# See them
+dstats small-strings
+
+# Who's retaining the string "foo"
+f = dfilter small-strings foo
+retainters f
+
+# Find stuff with specific field
+hasField = filter all :specificField
+stats closure hasField :specificField
+foo = follow hasField :specificField
+stats closure foo
+
+# Stop the closure search if going into specific files
+stats closure foo ^file1.dart ^file2.dart ^file3.dart
+