How do I merge two dictionaries in Smalltalk with a oneliner?

It depends on whether you want a third object or you prefer to include, say b, into a.

For the first case

c := Dictionary new.
a keysAndValuesDo: [:k :v | c at: k put: v].
b keysAndValuesDo: [:k :v | c at: k put: v].

For the second

b keysAndValuesDo: [:k :v | a at: k put: v].

Note also that the operation is not commutative, meaning that if the same key occurs in both dictionaries, the one that will survive in the result is the last added.

Another one:

Dictionary new addAll: a; addAll: b; yourself

Also works when a and b are other collections of Associations, as in your question without asDictionary. Works at least in Squeak.

As pointed out by @aka.nice in the comments, depending on your Smalltalk implementation it may have the side effect of sharing the Association objects with the input dictionaries. It certainly is that way in Squeak. If a and b have common keys, it may even modify one of the input dictionaries (in Squeak a) because first an existing Association is added to the new Dictionary's hashtable, and then this Association gets the value from the other input Dictionary assigned.

a := Dictionary newFrom: {#a -> 1}.
b := Dictionary newFrom: {#a -> 2}.
c := Dictionary new addAll: a; addAll: b; yourself.
{a at: #a. b at: #a. c at: #a}  "==> #(2 2 2) in Squeak"

In Squeak/Pharo you can simply use union:

a := {'a' -> 1} as: Dictionary.
b := {'b' -> 2} as: Dictionary.
c := a union: b.

-> a Dictionary('a'->1 'b'->2 )

Note that the elements of b will be chosen in case of overlapping keys

a := {'a' -> 1. 'c'->0} as:Dictionary.
b := {'b' -> 2. 'c'->7} as:Dictionary.
c := a union: b.

-> a Dictionary('a'->1 'b'->2 'c'->7 )

c := a, b works in Pharo (but not in Dolphin)

I think Leandro's and aka.nice's answer provide you with nice solutions. Both also mention that you are going to lose information when the key is same for both of the dictionaries.

I'm writing this in order to complement these answers. If you should need to keep the duplicate key -> value I would do it the following way:

a := {'a' -> 1. 'c'->3} as: Bag.
b := {'b' -> 2. 'c'->20} as: Bag.
c := a union: b.

Which will give you a Bag with Dictionary as contents:

Dictionary('b'->2->1 'a'->1->1 'c'->20->1 'c'->3->1)

(I'm using Smalltalk/X-jv)